publications | Jason L. Sanders

2025

ApJL
Observational Constraints of Radial Migration in the Galactic Disk Driven by the Slowing Bar

HanYuan Zhang, Vasily Belokurov, N. Wyn Evans, and 7 more authors

ApJL, Apr 2025

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Radial migration is an important dynamical effect that has reshaped the Galactic disk, but its origin has yet to be elucidated. In this work, we present evidence that resonant dragging by the corotation of a decelerating bar could be the main driver of radial migration in the Milky Way disk. Using a test particle simulation, we demonstrate this scenario explains the two distinct age–metallicity sequences observed in the solar vicinity: the plateauing upper sequence is interpreted as stars dragged outward by the expanding corotation of the decelerating bar and the steeper lower sequence as stars formed locally around the solar circle. The upper migrated sequence dominates at guiding radii around the current corotation radius of the bar, R \(∼\) 7 kpc, but rapidly dies away beyond this where the mechanism cannot operate. This behavior naturally explains the radial dependence of the [\(α\) /Fe]-bimodality, in particular the truncation of the high-[\(α\) /Fe] disk beyond the solar circle. Under our proposed radial migration scenario, we constrain the Milky Way bar’s pattern speed evolution using the age–metallicity distribution of stars currently trapped at corotation. We find the bar likely formed with an initial pattern speed of 60‑100 km s\(^‑1\) kpc\(^‑1\) and began decelerating 6‑8 Gyr ago at a rate of (where the quoted ranges include systematic uncertainties).
@article{2025ApJ...983L..10Z, author = {{Zhang}, HanYuan and {Belokurov}, Vasily and {Evans}, N. Wyn and {Sanders}, Jason L. and {Lu}, Yuxi(Lucy) and {Cao}, Chengye and {Myeong}, GyuChul and {Dillamore}, Adam M. and {Kane}, Sarah G. and {Li}, Zhao-Yu}, title = {{Observational Constraints of Radial Migration in the Galactic Disk Driven by the Slowing Bar}}, journal = {\apjl}, keywords = {Milky Way dynamics, Galactic bar, Milky Way evolution, Milky Way disk, Galaxy evolution, 1051, 2365, 1052, 1050, 594, Astrophysics of Galaxies}, year = {2025}, month = apr, volume = {983}, number = {1}, eid = {L10}, pages = {L10}, doi = {10.3847/2041-8213/adc261}, archiveprefix = {arXiv}, eprint = {2502.02642}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2025ApJ...983L..10Z}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Stellar streams from black hole-rich star clusters

Daniel Roberts, Mark Gieles, Denis Erkal, and 1 more author

MNRAS, Mar 2025

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Nearly a hundred progenitor-less, thin stellar streams have been discovered in the Milky Way, thanks to Gaia and related surveys. Most streams are believed to have formed from star clusters and it was recently proposed that extended star clusters - rich in stellar-mass black holes (BHs) - are efficient in creating streams. To better understand the nature of stream progenitors, we quantify the differences between streams originating from star clusters with and without BHs using direct N-body models and a new model for the density profiles of streams based on time-dependent escape rates from clusters: the Quantifying Stream Growth (QSG) model. QSG facilitates the rapid exploration of parameter space and provides an analytic framework to understand the impact of different star cluster properties and escape conditions on the structure of streams. Using these models it is found that, compared to streams from BH-free clusters on the same orbit, streams of BH-rich clusters: (1) are approximately 5 times more massive; (2) have a peak density 3 times closer to the cluster \(1 \rm Gyr\) post-evaporation (for orbits of Galactocentric radius \(rsim 10 \rm kpc\) ), and (3) have narrower peaks and more extended wings in their density profiles. We discuss other observable stream properties that are affected by the presence of BHs in their progenitor cluster, namely the width of the stream, its radial offset from the orbit, and the properties of the gap at the progenitor’s location. Our results provide a step towards using stellar streams to constrain the BH content of evaporated (globular) star clusters.
@article{2025MNRAS.538..454R, author = {{Roberts}, Daniel and {Gieles}, Mark and {Erkal}, Denis and {Sanders}, Jason L.}, title = {{Stellar streams from black hole-rich star clusters}}, journal = {\mnras}, keywords = {Astrophysics - Astrophysics of Galaxies}, year = {2025}, month = mar, volume = {538}, number = {1}, pages = {454-469}, doi = {10.1093/mnras/staf321}, archiveprefix = {arXiv}, eprint = {2402.06393}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2025MNRAS.538..454R}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
\aap
Variable stars in the VVV globular clusters: III. RR Lyrae stars in the inner Galactic globular clusters

Javier Alonso-Garcı́a, Leigh C. Smith, Jason L. Sanders, and 13 more authors

\aap, Mar 2025

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Context. High reddening near the Galactic plane hampers observations and proper characterization of the globular clusters (GCs) located toward the inner regions of the Milky Way. Aims. The VISTA Variables in the Vı́a Láctea (VVV) survey observed the Galactic bulge and adjacent disk for several years, providing multi-epoch, near-infrared images for 41 Galactic GCs. Detecting RR Lyrae variable stars belonging to these GCs will aid in their accurate parameterization. Methods. By fully leveraging the astrometric, photometric, and variability VVV catalogs, we searched for RR Lyrae stars associated with GCs. Our selection criteria, based on proper motions, proximity to the cluster centers, and distances inferred from their period- luminosity- metallicity relations, enable us to accurately identify the RR Lyrae population in these GCs and determine color excesses and distances to these poorly studied GCs in a homogeneous manner. Since the VVV catalogs cover from the innermost regions of the GCs to their outskirts, we can provide a comprehensive picture of the entire RR Lyrae population in these GCs. Results. We have discovered significant RR Lyrae populations in two highly reddened Galactic GCs: UKS 1 and VVV-CL160, previously unknown to host RR Lyrae stars. Additionally, we have detected one RR Lyrae candidate in each of Terzan 4 and Terzan 9, also new to RR Lyrae detection. We further confirm and increase the number of RR Lyrae stars detected in 22 other low-latitude Galactic GCs. The RR Lyrae distances place most of these GCs within the Galactic bulge, aligning well with the few GCs in our sample with reliable Gaia or Hubble Space Telescope measurements. However, most of the VVV GCs lack accurate Gaia distances, and literature distances are generally significantly smaller than those derived in this work. As a byproduct of our analysis, we have obtained the proper motions for all the VVV GCs, independently confirming Gaia results, except for two of the most reddened GCs: UKS 1 and 2MASS-GC02.
@article{2025A&A...695A..47A, author = {{Alonso-Garc{\'\i}a}, Javier and {Smith}, Leigh C. and {Sanders}, Jason L. and {Minniti}, Dante and {Catelan}, M{\'a}rcio and {Aravena Rojas}, Gonzalo and {Carballo-Bello}, Julio A. and {Fern{\'a}ndez-Trincado}, Jos{\'e} G. and {Ferreira Lopes}, Carlos E. and {Garro}, Elisa R. and {Guo}, Zhen and {Hempel}, Maren and {Lucas}, Philip W. and {Majaess}, Daniel and {Saito}, Roberto K. and {Vivas}, A. Katherina}, title = {{Variable stars in the VVV globular clusters: III. RR Lyrae stars in the inner Galactic globular clusters}}, journal = {\aap}, keywords = {stars: variables: RR Lyrae, globular clusters: general, globular clusters: individual: UKS 1, globular clusters: individual: VVV-CL160, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2025}, month = mar, volume = {695}, eid = {A47}, pages = {A47}, doi = {10.1051/0004-6361/202453558}, archiveprefix = {arXiv}, eprint = {2502.06504}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2025A&A...695A..47A}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Predicting metallicities and carbon abundances from Gaia XP spectra for (carbon-enhanced) metal-poor stars

Anke Ardern-Arentsen, Sarah G. Kane, Vasily Belokurov, and 4 more authors

MNRAS, Feb 2025

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Carbon-rich (C-rich) stars can be found at all metallicities and evolutionary stages. They are often the result of mass transfer from a companion, but some of the most metal-poor C-rich objects are likely carrying the imprint of the metal-free First Stars from birth. In this work, we employ a neural network to predict metallicities and carbon abundances for over 10 million stars with Gaia low-resolution XP spectra, down to \(\rm [Fe/H] = -3.0\) and up to \(\rm [C/Fe] ≈+2\) . We identify \(∼ 2000\) high-confidence bright (\(G 16\) ) carbon-enhanced metal-poor stars with \(\rm [Fe/H] -2.0\) and \(\rm [C/Fe] +0.7\) . The majority of our C-rich candidates have \(\rm [Fe/H] -2.0\) and are expected to be binary mass-transfer products, supported by high barium abundances in the GALAH (GALactic Archaeology with HERMES) survey and/or their Gaia Renormalised Unit Weight Error (RUWE) and radial velocity variations. We confirm previous findings of an increase in C-rich stars with decreasing metallicity, adopting a definition of \(3σ\) outliers from the [C/Fe] distribution, although our frequency appears to flatten for \(-3.0 \rm [Fe/H] -2.0\) at a level of \(6\!\!-\!\!7 \rm per\,cent\) . We also find that the fraction of C-rich stars is low among globular cluster stars (connected to their lower binary fraction), and that it decreases for field stars more tightly bound to the Milky Way. We interpret these last results as evidence that disrupted globular clusters contribute more in the inner Galaxy, supporting previous work. Homogeneous samples such as these are key to understanding the full population properties of C-rich stars, and this is just the beginning.
@article{2025MNRAS.537.1984A, author = {{Ardern-Arentsen}, Anke and {Kane}, Sarah G. and {Belokurov}, Vasily and {Matsuno}, Tadafumi and {Montelius}, Martin and {Monty}, Stephanie and {Sanders}, Jason L.}, title = {{Predicting metallicities and carbon abundances from Gaia XP spectra for (carbon-enhanced) metal-poor stars}}, journal = {\mnras}, keywords = {Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2025}, month = feb, volume = {537}, number = {2}, pages = {1984-2002}, doi = {10.1093/mnras/staf096}, archiveprefix = {arXiv}, eprint = {2410.11077}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2025MNRAS.537.1984A}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
VIRAC2: NIR astrometry and time series photometry for 500M+ stars from the VVV and VVVX surveys

Leigh C. Smith, Philip W. Lucas, Sergey E. Koposov, and 11 more authors

MNRAS, Feb 2025

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We present VIRAC2, a catalogue of positions, proper motions, parallaxes and Z, Y, J, H, and \(K_s\) near- infrared photometric time series of 545 346 537 unique stars. The catalogue is based on a point spread function fitting reduction of nearly a decade of VISTA VVV and VVVX images, which cover \(560 \rm deg^2\) of the Southern Galactic plane and bulge. The catalogue is complete at the \( 90\) per cent level for \(11 K_s \rm mag 16\) sources, but extends to \(K_s≈17.5\) mag in most fields. Astrometric performance for \(11 K_s \rm mag 14\) sources is typically \(≈0.37 \rm mas yr^-1\) per dimension for proper motion, and \(1 \rm mas\) for parallax. At \(K_s=16\) the equivalent values are around \(1.5 \rm mas yr^-1\) and \(5 \rm mas\) . These uncertainties are validated against Gaia DR3 and Hubble Space Telescope astrometry. The complete catalogues are available via the ESO archive. We perform an initial search of the catalogue for nearby ultracool dwarf candidates. In total, we find 26 new sources whose parallaxes place them within 50 parsecs of the Sun. Among them we find two high-confidence T dwarfs and a number of other sources that appear to lie close to the L/T transition.
@article{2025MNRAS.536.3707S, author = {{Smith}, Leigh C. and {Lucas}, Philip W. and {Koposov}, Sergey E. and {Gonzalez-Fernandez}, Carlos and {Alonso-Garc{\'\i}a}, Javier and {Minniti}, Dante and {Sanders}, Jason L. and {Bedin}, Luigi R. and {Belokurov}, Vasily and {Evans}, N. Wyn and {Hempel}, Maren and {Ivanov}, Valentin D. and {Kurtev}, Radostin G. and {Saito}, Roberto K.}, title = {{VIRAC2: NIR astrometry and time series photometry for 500M+ stars from the VVV and VVVX surveys}}, journal = {\mnras}, keywords = {Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2025}, month = feb, volume = {536}, number = {4}, pages = {3707-3738}, doi = {10.1093/mnras/stae2797}, archiveprefix = {arXiv}, eprint = {2501.06295}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2025MNRAS.536.3707S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJ
LMC Calls, Milky Way Halo Answers: Disentangling the Effects of the MW–LMC Interaction on Stellar Stream Populations

Richard A. N. Brooks, Nicolás Garavito-Camargo, Kathryn V. Johnston, and 3 more authors

ApJ, Jan 2025

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The infall of the LMC into the Milky Way (MW) has dynamical implications throughout the MW’s dark matter halo. We study the impact of this merger on the statistical properties of populations of simulated stellar streams. Specifically, we investigate the radial and on-sky angular dependence of stream perturbations caused by the direct effect of stream–LMC interactions and/or the response of the MW dark matter halo. We use a time-evolving MW–LMC simulation described by basis function expansions to simulate streams. We quantify the degree of perturbation using a set of stream property statistics including the misalignment of proper motions with the stream track. In the outer halo, direct stream–LMC interactions produce a statistically significant effect, boosting the fraction of misaligned proper motions by \(∼\) 25% relative to the model with no LMC. Moreover, there is on-sky angular dependence of stream perturbations: the highest fractions of perturbed streams coincide with the same on-sky quadrant as the present-day LMC location. In the inner halo, the MW halo dipole response primarily drives stream perturbations, but it remains uncertain whether this is a detectable signature distinct from the LMC’s influence. For the fiducial MW–LMC model, we find agreement between the predicted fraction of streams with significantly misaligned proper motions, , and Dark Energy Survey data. Finally, we predict this fraction for the Rubin Observatory Legacy Survey of Space and Time (LSST) footprint. Using LSST data will improve our constraints on dark matter models and LMC properties, as it is sensitive to both.
@article{2025ApJ...978...79B, author = {{Brooks}, Richard A.~N. and {Garavito-Camargo}, Nicol{\'a}s and {Johnston}, Kathryn V. and {Price-Whelan}, Adrian M. and {Sanders}, Jason L. and {Lilleengen}, Sophia}, title = {{LMC Calls, Milky Way Halo Answers: Disentangling the Effects of the MW{\textendash}LMC Interaction on Stellar Stream Populations}}, journal = {\apj}, keywords = {Milky Way dynamics, Large Magellanic Cloud, Stellar streams, 1051, 903, 2166, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2025}, month = jan, volume = {978}, number = {1}, eid = {79}, pages = {79}, doi = {10.3847/1538-4357/ad93a7}, archiveprefix = {arXiv}, eprint = {2410.02574}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2025ApJ...978...79B}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2024

MNRAS
Deciphering the Milky Way disc formation time encrypted in the bar chrono-kinematics

Hanyuan Zhang, Vasily Belokurov, N. Wyn Evans, and 3 more authors

MNRAS, Dec 2024

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We present a novel method to constrain the formation time of the Milky Way disc using the chrono-kinematic signatures of the inner Galaxy. We construct an O-rich Mira variable sample from the Gaia long-period variable catalogue to study the kinematic behaviour of stars with different ages in the inner Galaxy. From the Auriga suite of cosmological zoom-in simulations, we find that the age of the oldest stellar population with imprints of the bar in density and kinematics matches the disc spin-up epoch. This is because stars born before the spin-up show insufficient rotation and are not kinematically cold enough to be efficiently trapped by the bar. We find that the bar kinematic signature disappears for Mira variables with a period shorter than 190 d. Using the period-age relation of Mira variables, we constrain the spin-up epoch of the Milky Way to be younger than \(∼11-12\) Gyr (redshift \(∼3\) ). We also discuss and compare our method and result to other evidence of the Milky Way spin-up epoch under the context of a realistic age uncertainty. Age uncertainty leads to an overestimation of the disc formation time when performing backward modelling. Our constrain of the spin-up epoch is independent from previous studies because it relies on the kinematics of the inner Galaxy instead of the solar vicinity.
@article{2024MNRAS.535.2873Z, author = {{Zhang}, Hanyuan and {Belokurov}, Vasily and {Evans}, N. Wyn and {Li}, Zhao-Yu and {Sanders}, Jason L. and {Ardern-Arentsen}, Anke}, title = {{Deciphering the Milky Way disc formation time encrypted in the bar chrono-kinematics}}, journal = {\mnras}, keywords = {Astrophysics - Astrophysics of Galaxies}, year = {2024}, month = dec, volume = {535}, number = {3}, pages = {2873-2888}, doi = {10.1093/mnras/stae2546}, archiveprefix = {arXiv}, eprint = {2408.16815}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.535.2873Z}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Kinematics and dynamics of the Galactic bar revealed by Gaia long-period variables

Hanyuan Zhang, Vasily Belokurov, N. Wyn Evans, and 2 more authors

MNRAS, Sep 2024

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We use low-amplitude long period variable (LA-LPV) candidates in Gaia DR3 to trace the kinematics and dynamics of the Milky Way bar. LA-LPVs, like other LPVs, are intrinsically bright and follow a tight period-luminosity relation, but unlike e.g. Mira variables, their radial velocity measurements are reliable due to their smaller pulsation amplitudes. We supplement the Gaia astrometric and radial velocity measurements with distance moduli assigned using a period-luminosity relation to acquire full 6D phase space information. The assigned distances are validated by comparing to geometric distances and StarHorse distances, which shows biases less than \(∼5 \rm per\ cent\) . Our sample provides an unprecedented panoramic picture of the inner Galaxy with minimal selection effects. We map the kinematics of the inner Milky Way and find a significant kinematic signature corresponding to the Galactic bar. We measure the pattern speed of the Galactic bar using the continuity equation and find \(Ω_\rm b=34.1\pm 2.4\) km s\(^-1\) kpc\(^-1\) . We develop a simple robust and potential-independent method to measure the dynamical length of the bar using only kinematics and find \(R_\rm b∼4.0\) kpc. We validate both measurements using N-body simulations. Assuming knowledge of the gravitational potential of the inner Milky Way, we analyse the orbital structure of the Galactic bar using orbital frequency ratios. The \(x_1\) orbits are the dominant bar-supporting orbital family in our sample. Amongst the selected bar stars, the \(x_1 v_1\) or ’banana’ orbits constitute a larger fraction (\(∼15 \rm per cent\) ) than other orbital families in the bar, implying that they are the dominant family contributing to the Galactic X-shape, although contributions from other orbital families are also present.
@article{2024MNRAS.533.3395Z, author = {{Zhang}, Hanyuan and {Belokurov}, Vasily and {Evans}, N. Wyn and {Kane}, Sarah G. and {Sanders}, Jason L.}, title = {{Kinematics and dynamics of the Galactic bar revealed by Gaia long-period variables}}, journal = {\mnras}, keywords = {Astrophysics - Astrophysics of Galaxies}, year = {2024}, month = sep, volume = {533}, number = {3}, pages = {3395-3414}, doi = {10.1093/mnras/stae2023}, archiveprefix = {arXiv}, eprint = {2406.06678}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.533.3395Z}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The ratio of [Eu/\(α\) ] differentiates accreted/in situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters

Stephanie Monty, Vasily Belokurov, Jason L. Sanders, and 7 more authors

MNRAS, Sep 2024

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We combine stellar orbits with the abundances of the heavy, r-process element europium and the light, \(α\) -element, silicon to separate in situ and accreted populations in the Milky Way (MW) across all metallicities. At high orbital energy, the accretion- dominated halo shows elevated values of [Eu/Si], while at lower energies, where many of the stars were born in situ, the levels of [Eu/Si] are lower. These systematically different levels of [Eu/Si] in the MW and the accreted halo imply that the scatter in [Eu/\(α\) ] within a single galaxy is smaller than previously thought. At the lowest metallicities, we find that both accreted and in situ populations trend down in [Eu/Si], consistent with enrichment via neutron star mergers. Through compiling a large data set of abundances for 54 globular clusters (GCs), we show that differences in [Eu/Si] extend to populations of in situ/accreted GCs. We interpret this consistency as evidence that in r-process elements GCs trace the star formation history of their hosts, motivating their use as sub-Gyr timers of galactic evolution. Furthermore, fitting the trends in [Eu/Si] using a simple galactic chemical evolution model, we find that differences in [Eu/Si] between accreted and in situ MW field stars cannot be explained through star formation efficiency alone. Finally, we show that the use of [Eu/Si] as a chemical tag between GCs and their host galaxies extends beyond the Local Group, to the halo of M31 - potentially offering the opportunity to do Galactic Archaeology in an external galaxy.
@article{2024MNRAS.533.2420M, author = {{Monty}, Stephanie and {Belokurov}, Vasily and {Sanders}, Jason L. and {Hansen}, Terese T. and {Sakari}, Charli M. and {McKenzie}, Madeleine and {Myeong}, GyuChul and {Davies}, Elliot Y. and {Ardern-Arentsen}, Anke and {Massari}, Davide}, title = {{The ratio of [Eu/{\(\alpha\)\ }] differentiates accreted/in situ Milky Way stars across metallicities, as indicated by both field stars and globular clusters}}, journal = {\mnras}, keywords = {Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2024}, month = sep, volume = {533}, number = {2}, pages = {2420-2440}, doi = {10.1093/mnras/stae1895}, archiveprefix = {arXiv}, eprint = {2405.08963}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.533.2420M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Action and energy clustering of stellar streams in deforming Milky Way dark matter haloes

Richard A. N. Brooks, Jason L. Sanders, Sophia Lilleengen, and 2 more authors

MNRAS, Aug 2024

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We investigate the non-adiabatic effect of time-dependent deformations in the Milky Way (MW) halo potential on stellar streams. Specifically, we consider the MW’s response to the infall of the Large Magellanic Cloud (LMC) and how this impacts our ability to recover the spherically averaged MW mass profile from observation using stream actions. Previously, action clustering methods have only been applied to static or adiabatic MW systems to constrain the properties of the host system. We use a time- evolving MW-LMC simulation described by basis function expansions. We find that for streams with realistic observational uncertainties on shorter orbital periods and without close encounters with the LMC, e.g. GD-1, the radial action distribution is sufficiently clustered to locally recover the spherical MW mass profile across the stream radial range within a \(2σ\) confidence interval determined using a Fisher information approach. For streams with longer orbital periods and close encounters with the LMC, e.g. Orphan-Chenab (OC), the radial action distribution disperses as the MW halo has deformed non-adiabatically. Hence, for OC streams generated in potentials that include an MW halo with any deformations, action clustering methods will fail to recover the spherical mass profile within a \(2σ\) uncertainty. Finally, we investigate whether the clustering of stream energies can provide similar constraints. Surprisingly, we find for OC-like streams, the recovered spherically averaged mass profiles demonstrate less sensitivity to the time-dependent deformations in the potential.
@article{2024MNRAS.532.2657B, author = {{Brooks}, Richard A.~N. and {Sanders}, Jason L. and {Lilleengen}, Sophia and {Petersen}, Michael S. and {Pontzen}, Andrew}, title = {{Action and energy clustering of stellar streams in deforming Milky Way dark matter haloes}}, journal = {\mnras}, keywords = {Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2024}, month = aug, volume = {532}, number = {2}, pages = {2657-2673}, doi = {10.1093/mnras/stae1565}, archiveprefix = {arXiv}, eprint = {2401.11990}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.532.2657B}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJ
Hunting for Polluted White Dwarfs and Other Treasures with Gaia XP Spectra and Unsupervised Machine Learning

Malia L. Kao, Keith Hawkins, Laura K. Rogers, and 5 more authors

ApJ, Aug 2024

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White dwarfs (WDs) polluted by exoplanetary material provide the unprecedented opportunity to directly observe the interiors of exoplanets. However, spectroscopic surveys are often limited by brightness constraints, and WDs tend to be very faint, making detections of large populations of polluted WDs difficult. In this paper, we aim to increase considerably the number of WDs with multiple metals in their atmospheres. Using 96,134 WDs with Gaia DR3 BP/RP (XP) spectra, we constructed a 2D map using an unsupervised machine-learning technique called Uniform Manifold Approximation and Projection (UMAP) to organize the WDs into identifiable spectral regions. The polluted WDs are among the distinct spectral groups identified in our map. We have shown that this selection method could potentially increase the number of known WDs with five or more metal species in their atmospheres by an order of magnitude. Such systems are essential for characterizing exoplanet diversity and geology.
@article{2024ApJ...970..181K, author = {{Kao}, Malia L. and {Hawkins}, Keith and {Rogers}, Laura K. and {Bonsor}, Amy and {Dunlap}, Bart H. and {Sanders}, Jason L. and {Montgomery}, M.~H. and {Winget}, D.~E.}, title = {{Hunting for Polluted White Dwarfs and Other Treasures with Gaia XP Spectra and Unsupervised Machine Learning}}, journal = {\apj}, keywords = {Gaia, White dwarf stars, DZ stars, 2360, 1799, 1848, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Earth and Planetary Astrophysics, Computer Science - Machine Learning}, year = {2024}, month = aug, volume = {970}, number = {2}, eid = {181}, pages = {181}, doi = {10.3847/1538-4357/ad5d6e}, archiveprefix = {arXiv}, eprint = {2405.17667}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024ApJ...970..181K}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

PASJ

JASMINE: Near-infrared astrometry and time-series photometry science

Daisuke Kawata, Hajime Kawahara, Naoteru Gouda, and 86 more authors

PASJ, Jun 2024

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The Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE) is a planned M-class science space mission by the Institute of Space and Astronautical Science, the Japan Aerospace Exploration Agency. JASMINE has two main science goals. One is Galactic archaeology with a Galactic Center survey, which aims to reveal the Milky Way’s central core structure and formation history from Gaia-level (\(∼\) 25 \(μ \) as) astrometry in the near-infrared (NIR) H\(_w\) band (1.0-1.6 \(μ \) m). The other is an exoplanet survey, which aims to discover transiting Earth-like exoplanets in the habitable zone from NIR time-series photometry of M dwarfs when the Galactic Center is not accessible. We introduce the mission, review many science objectives, and present the instrument concept. JASMINE will be the first dedicated NIR astrometry space mission and provide precise astrometric information on the stars in the Galactic Center, taking advantage of the significantly lower extinction in the NIR. The precise astrometry is obtained by taking many short-exposure images. Hence, the JASMINE Galactic Center survey data will be valuable for studies of exoplanet transits, asteroseismology, variable stars, and microlensing studies, including discovery of (intermediate-mass) black holes. We highlight a swath of such potential science, and also describe synergies with other missions.

@article{2024PASJ...76..386K,
  author = {{Kawata}, Daisuke and {Kawahara}, Hajime and {Gouda}, Naoteru and {Secrest}, Nathan J. and {Kano}, Ryouhei and {Kataza}, Hirokazu and {Isobe}, Naoki and {Ohsawa}, Ryou and {Usui}, Fumihiko and {Yamada}, Yoshiyuki and {Graham}, Alister W. and {Pettitt}, Alex R. and {Asada}, Hideki and {Baba}, Junichi and {Bekki}, Kenji and {Dorland}, Bryan N. and {Fujii}, Michiko and {Fukui}, Akihiko and {Hattori}, Kohei and {Hirano}, Teruyuki and {Kamizuka}, Takafumi and {Kashima}, Shingo and {Kawanaka}, Norita and {Kawashima}, Yui and {Klioner}, Sergei A. and {Kodama}, Takanori and {Koshimoto}, Naoki and {Kotani}, Takayuki and {Kuzuhara}, Masayuki and {Levine}, Stephen E. and {Majewski}, Steven R. and {Masuda}, Kento and {Matsunaga}, Noriyuki and {Miyakawa}, Kohei and {Miyoshi}, Makoko and {Morihana}, Kumiko and {Nishi}, Ryoichi and {Notsu}, Yuta and {Omiya}, Masashi and {Sanders}, Jason and {Tanikawa}, Ataru and {Tsujimoto}, Masahiro and {Yano}, Taihei and {Aizawa}, Masataka and {Arimatsu}, Ko and {Biermann}, Michael and {Boehm}, Celine and {Chiba}, Masashi and {Debattista}, Victor P. and {Gerhard}, Ortwin and {Hirabayashi}, Masayuki and {Hobbs}, David and {Ikenoue}, Bungo and {Izumiura}, Hideyuki and {Jordi}, Carme and {Kohara}, Naoki and {L{\"o}ffler}, Wolfgang and {Luri}, Xavier and {Mase}, Ichiro and {Miglio}, Andrea and {Mitsuda}, Kazuhisa and {Newswander}, Trent and {Nishiyama}, Shogo and {Obuchi}, Yoshiyuki and {Ootsubo}, Takafumi and {Ouchi}, Masami and {Ozaki}, Masanobu and {Perryman}, Michael and {Prusti}, Timo and {Ramos}, Pau and {Read}, Justin I. and {Rich}, R. Michael and {Sch{\"o}nrich}, Ralph and {Shikauchi}, Minori and {Shimizu}, Risa and {Suematsu}, Yoshinori and {Tada}, Shotaro and {Takahashi}, Aoi and {Tatekawa}, Takayuki and {Tatsumi}, Daisuke and {Tsujimoto}, Takuji and {Tsuzuki}, Toshihiro and {Urakawa}, Seitaro and {Uraguchi}, Fumihiro and {Utsunomiya}, Shin and {Van Eylen}, Vincent and {van Leeuwen}, Floor and {Wada}, Takehiko and {Walton}, Nicholas A.},
  title = {{JASMINE: Near-infrared astrometry and time-series photometry science}},
  journal = {\pasj},
  keywords = {astrometry, Galaxy: Center, infrared: planetary systems, space vehicles: instruments, techniques: photometric, Astrophysics - Instrumentation and Methods for Astrophysics, Astrophysics - Earth and Planetary Astrophysics, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics},
  year = {2024},
  month = jun,
  volume = {76},
  number = {3},
  pages = {386-425},
  doi = {10.1093/pasj/psae020},
  archiveprefix = {arXiv},
  eprint = {2307.05666},
  primaryclass = {astro-ph.IM},
  adsurl = {https://ui.adsabs.harvard.edu/abs/2024PASJ...76..386K},
  adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

MNRAS
Stellar atmospheric parameters from Gaia BP/RP spectra using uncertain neural networks

Connor P. Fallows, and Jason L. Sanders

MNRAS, Jun 2024

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With the plentiful information available in the Gaia BP/RP spectra, there is significant scope for applying discriminative models to extract stellar atmospheric parameters and abundances. We describe an approach to leverage an ’Uncertain Neural Network’ model trained on APOGEE data to provide high-quality predictions with robust estimates for per-prediction uncertainty. We report median formal uncertainties of 0.068 dex, 69.1 K, 0.14 dex, 0.031 dex, 0.040 dex, and 0.029 dex for [Fe/H], T\(_eff\) , log g, [C/Fe], [N/Fe], and [\(α\) /M], respectively. We validate these predictions against our APOGEE training data, LAMOST, and Gaia GSP-PHOT stellar parameters, and see a strong correlation between our predicted parameters and those derived from these surveys. We investigate the information content of the spectra by considering the ’attention’ our model pays to different spectral features compared to expectations from synthetic spectra calculations. Our model’s predictions are applied to the Gaia data set, and we produce a publicly available catalogue of our model’s predictions.
@article{2024MNRAS.531.2126F, author = {{Fallows}, Connor P. and {Sanders}, Jason L.}, title = {{Stellar atmospheric parameters from Gaia BP/RP spectra using uncertain neural networks}}, journal = {\mnras}, keywords = {Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies, Astrophysics - Instrumentation and Methods for Astrophysics}, year = {2024}, month = jun, volume = {531}, number = {1}, pages = {2126-2147}, doi = {10.1093/mnras/stae1303}, archiveprefix = {arXiv}, eprint = {2405.10699}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.531.2126F}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The epoch of the Milky Way’s bar formation: dynamical modelling of Mira variables in the nuclear stellar disc

Jason L. Sanders, Daisuke Kawata, Noriyuki Matsunaga, and 4 more authors

MNRAS, May 2024

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A key event in the history of the Milky Way is the formation of the bar. This event affects the subsequent structural and dynamical evolution of the entire Galaxy. When the bar formed, gas was likely rapidly funnelled to the centre of the Galaxy settling in a star-forming nuclear disc. The Milky Way bar formation can then be dated by considering the age distribution of the oldest stars in the formed nuclear stellar disc. In this highly obscured and crowded region, reliable age tracers are limited, but bright, high-amplitude Mira variables make useful age indicators as they follow a period-age relation. We fit dynamical models to the proper motions of a sample of Mira variables in the Milky Way’s nuclear stellar disc region. Weak evidence for inside-out growth and both radial and vertical dynamical heating with time of the nuclear stellar disc is presented, suggesting that the nuclear stellar disc is dynamically well-mixed. Furthermore, for Mira variables around a \(∼\) 350-d period, there is a clear transition from nuclear stellar disc-dominated kinematics to background bar-bulge-dominated kinematics. Using a Mira variable period-age relation calibrated in the solar neighbourhood, this suggests the nuclear stellar disc formed in a significant burst in star formation \((8\pm 1) \mathrmGyr\) ago, although the data are also weakly consistent with a more gradual formation of the nuclear stellar disc at even earlier epochs. This implies a relatively early formation time for the Milky Way bar (\(rsim 8 \mathrmGyr\) ), which has implications for the growth and state of the young Milky Way and its subsequent history.
@article{2024MNRAS.530.2972S, author = {{Sanders}, Jason L. and {Kawata}, Daisuke and {Matsunaga}, Noriyuki and {Sormani}, Mattia C. and {Smith}, Leigh C. and {Minniti}, Dante and {Gerhard}, Ortwin}, title = {{The epoch of the Milky Way's bar formation: dynamical modelling of Mira variables in the nuclear stellar disc}}, journal = {\mnras}, keywords = {stars: AGB, stars: variables: general, Galaxy: evolution, Galaxy: formation, Galaxy: kinematics and dynamics, Galaxy: nucleus, Astrophysics - Astrophysics of Galaxies}, year = {2024}, month = may, volume = {530}, number = {3}, pages = {2972-2993}, doi = {10.1093/mnras/stae711}, archiveprefix = {arXiv}, eprint = {2311.00035}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.530.2972S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Nuclear rings are the inner edge of a gap around the Lindblad Resonance

Mattia C. Sormani, Emanuele Sobacchi, and Jason L. Sanders

MNRAS, Mar 2024

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Gaseous nuclear rings are large-scale coherent structures commonly found at the centres of barred galaxies. We propose that they are an accumulation of gas at the inner edge of an extensive gap that forms around the Inner Lindblad Resonance (ILR). The gap initially opens because the bar potential excites strong trailing waves near the ILR, which remove angular momentum from the gas disc and transport the gas inwards. The gap then widens because the bar potential continuously excites trailing waves at the inner edge of the gap, which remove further angular momentum, moving the edge further inwards until it stops at a distance of several wavelengths from the ILR. The gas accumulating at the inner edge of the gap forms the nuclear ring. The speed at which the gap edge moves and its final distance from the ILR strongly depend on the sound speed, explaining the puzzling dependence of the nuclear ring radius on the sound speed in simulations.
@article{2024MNRAS.528.5742S, author = {{Sormani}, Mattia C. and {Sobacchi}, Emanuele and {Sanders}, Jason L.}, title = {{Nuclear rings are the inner edge of a gap around the Lindblad Resonance}}, journal = {\mnras}, keywords = {galaxies: bulges, galaxies: kinematics and dynamics, galaxies: ISM, Astrophysics - Astrophysics of Galaxies}, year = {2024}, month = mar, volume = {528}, number = {4}, pages = {5742-5762}, doi = {10.1093/mnras/stae082}, archiveprefix = {arXiv}, eprint = {2309.14093}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.528.5742S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The most variable VVV sources: eruptive protostars, dipping giants in the nuclear disc and others

P. W. Lucas, L. C. Smith, Z. Guo, and 16 more authors

MNRAS, Feb 2024

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We have performed a comprehensive search of a VISTA Variables in the Via Lactea (VVV) data base of 9.5 yr light curves for variable sources with \(∆\) K\(_s\) \(≥\) 4 mag, aiming to provide a large sample of high amplitude eruptive young stellar objects (YSOs) and detect unusual or new types of infrared variable source. We find 222 variable or transient sources in the Galactic bulge and disc, most of which are new discoveries. The sample mainly comprises novae, YSOs, microlensing events, Long Period Variable stars (LPVs), and a few rare or unclassified sources. Additionally, we report the discovery of a significant population of aperiodic late-type giant stars suffering deep extinction events, strongly clustered in the Nuclear Disc of the Milky Way. We suggest that these are metal-rich stars in which radiatively driven mass loss has been enhanced by super-solar metallicity. Among the YSOs, 32/40 appear to be undergoing episodic accretion. Long-lasting YSO eruptions have a typical rise time of \(∼\) 2 yr, somewhat slower than the 6-12 month time-scale seen in the few historical events observed on the rise. The outburst durations are usually at least 5 yr, somewhat longer than many lower amplitude VVV events detected previously. The light curves are diverse in nature, suggesting that multiple types of disc instability may occur. Eight long- duration extinction events are seen wherein the YSO dims for a year or more, attributable to inner disc structure. One binary YSO in NGC 6530 displays periodic extinction events (P=59 d) similar to KH 15D.
@article{2024MNRAS.528.1789L, author = {{Lucas}, P.~W. and {Smith}, L.~C. and {Guo}, Z. and {Contreras Pe{\~n}a}, C. and {Minniti}, D. and {Miller}, N. and {Alonso-Garc{\'\i}a}, J. and {Catelan}, M. and {Borissova}, J. and {Saito}, R.~K. and {Kurtev}, R. and {Navarro}, M.~G. and {Morris}, C. and {Muthu}, H. and {Froebrich}, D. and {Ivanov}, V.~D. and {Bayo}, A. and {Caratti o Garatti}, A. and {Sanders}, J.~L.}, title = {{The most variable VVV sources: eruptive protostars, dipping giants in the nuclear disc and others}}, journal = {\mnras}, keywords = {stars: pre-main sequence, stars: protostars, stars: variables: T Tauri, Herbig Ae/Be, infrared: stars - stars: mass-loss - stars: AGB and post-AGB, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2024}, month = feb, volume = {528}, number = {2}, pages = {1789-1822}, doi = {10.1093/mnras/stad3929}, archiveprefix = {arXiv}, eprint = {2401.14471}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.528.1789L}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
From particles to orbits: precise dark matter density profiles using dynamical information

Claudia Muni, Andrew Pontzen, Jason L. Sanders, and 3 more authors

MNRAS, Jan 2024

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We introduce a new method to calculate dark matter halo density profiles from simulations. Each particle is ’smeared’ over its orbit to obtain a dynamical profile that is averaged over a dynamical time, in contrast to the traditional approach of binning particles based on their instantaneous positions. The dynamical and binned profiles are in good agreement, with the dynamical approach showing a significant reduction in Poisson noise in the innermost regions. We find that the inner cusps of the new dynamical profiles continue inward all the way to the softening radius, reproducing the central density profile of higher resolution simulations within the 95 per cent confidence intervals, for haloes in virial equilibrium. Folding in dynamical information thus provides a new approach to improve the precision of dark matter density profiles at small radii, for minimal computational cost. Our technique makes two key assumptions that the halo is in equilibrium (phase mixed) and the potential is spherically symmetric. We discuss why the method is successful despite strong violations of spherical symmetry in the centres of haloes, and explore how substructures disturb equilibrium at large radii.
@article{2024MNRAS.527.9250M, author = {{Muni}, Claudia and {Pontzen}, Andrew and {Sanders}, Jason L. and {Rey}, Martin P. and {Read}, Justin I. and {Agertz}, Oscar}, title = {{From particles to orbits: precise dark matter density profiles using dynamical information}}, journal = {\mnras}, keywords = {galaxies: haloes, galaxies: kinematics and dynamics, dark matter, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2024}, month = jan, volume = {527}, number = {3}, pages = {9250-9262}, doi = {10.1093/mnras/stad3835}, archiveprefix = {arXiv}, eprint = {2310.00044}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2024MNRAS.527.9250M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2023

MNRAS
The period-luminosity relation for Mira variables in the Milky Way using Gaia DR3: a further distance anchor for H\(_0\)

Jason L. Sanders

MNRAS, Aug 2023

Abs ADS arXiv pdf Bib DOI

Gaia DR3 parallaxes are used to calibrate preliminary period-luminosity relations of O-rich Mira variables in the 2MASS J, H, and K\(_s\) bands using a probabilistic model accounting for variations in the parallax zero-point and underestimation of the parallax uncertainties. The derived relations are compared to those measured for the Large and Small Magellanic Clouds, the Sagittarius dwarf spheroidal galaxy, globular cluster members, and the subset of Milky Way Mira variables with VLBI parallaxes. The Milky Way linear JHK\(_s\) relations are slightly steeper and thus fainter at short period than the corresponding LMC relations, suggesting population effects in the near-infrared are perhaps larger than previous observational works have claimed. Models of the Gaia astrometry for the Mira variables suggest that, despite the intrinsic photocentre wobble and use of mean photometry in the astrometric solution of the current data reduction, the recovered parallaxes should be on average unbiased but with underestimated uncertainties for the nearest stars. The recommended Gaia EDR3 parallax zero-point corrections evaluated at \(ν_\mathrmeff=1.25 μ\mathrmm^-1\) require minimal (\(≲5 μ\mathrmas\) ) corrections for redder five-parameter sources, but overcorrect the parallaxes for redder six-parameter sources, and the parallax uncertainties are underestimated at most by a factor \(∼\) 1.6 at \(G≈12.5 \mathrmmag\) . The derived period-luminosity relations are used as anchors for the Mira variables in the Type Ia host galaxy NGC 1559 to find \(H_0=(73.7\pm 4.4) \mathrmkm\,s^-1 \mathrmMpc^-1\) .
@article{2023MNRAS.523.2369S, author = {{Sanders}, Jason L.}, title = {{The period-luminosity relation for Mira variables in the Milky Way using Gaia DR3: a further distance anchor for H\(_{0}\)\ }}, journal = {\mnras}, keywords = {stars: AGB, stars: distances, stars: variables: general, cosmological parameters, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - Solar and Stellar Astrophysics}, year = {2023}, month = aug, volume = {523}, number = {2}, pages = {2369-2398}, doi = {10.1093/mnras/stad1431}, archiveprefix = {arXiv}, eprint = {2304.01671}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.523.2369S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
The epoch of bar formation in the Milky Way

Jason Sanders

In Galactic Bars: Driving and Decoding Galaxy Evolution, Jul 2023

Abs ADS Bib DOI

A key event in the life of the Milky Way is the formation of the bar. The Milky Way bar has restructured the disc, been slowed by the dark matter halo and funnelled gas towards the Galactic Centre. It is then important to know the period of time over which these processes have been effective. Although the bar is formed from stars that are older than the dynamical age of the bar, the nuclear stellar disc (NSD) formation was likely coincident with bar formation. The NSD region is highly crowded and obscured, but Mira variables make bright age tracers of this region. I will describe work on discovering Mira variables in this region of the Galaxy using the VVV survey, the calibration of the Mira variable period-age relation using nearby counterparts observed by Gaia and the dynamical modelling of the NSD Mira variable proper motion distribution. The dynamical modelling reveals the NSD likely formed around \(∼\) 7-8 Gyr ago. I discuss how this event fits into the timeline of events in the early Milky Way.}
@inproceedings{2023gbdd.confE..56S, author = {{Sanders}, Jason}, title = {{The epoch of bar formation in the Milky Way}}, booktitle = {Galactic Bars: Driving and Decoding Galaxy Evolution}, year = {2023}, month = jul, eid = {56}, pages = {56}, doi = {10.5281/zenodo.8204625}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023gbdd.confE..56S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Hunting for C-rich long-period variable stars in the Milky Way’s bar-bulge using unsupervised classification of Gaia BP/RP spectra

Jason L. Sanders, and Noriyuki Matsunaga

MNRAS, May 2023

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The separation of oxygen- and carbon-rich asymptotic giant branch sources is crucial for their accurate use as local and cosmological distance and age/metallicity indicators. We investigate the use of unsupervised learning algorithms for classifying the chemistry of long-period variables from Gaia DR3’s BP/RP spectra. Even in the presence of significant interstellar dust, the spectra separate into two groups attributable to O-rich and C-rich sources. Given these classifications, we utilize a supervised approach to separate O-rich and C-rich sources without blue and red photometers (BP/RP) spectra but instead given broadband optical and infrared photometry finding a purity of our C-rich classifications of around 95 per cent. We test and validate the classifications against other advocated colour-colour separations based on photometry. Furthermore, we demonstrate the potential of BP/RP spectra for finding S-type stars or those possibly symbiotic sources with strong emission lines. Although our classification suggests the Galactic bar-bulge is host to very few C-rich long- period variable stars, we do find a small fraction of C-rich stars with periods \( 250 \mathrmday\) that are spatially and kinematically consistent with bar-bulge membership. We argue the combination of the observed number, the spatial alignment, the kinematics, and the period distribution disfavour young metal- poor star formation scenarios either in situ or in an accreted host, and instead, these stars are highly likely to be the result of binary evolution and the evolved versions of blue straggler stars already observed in the bar-bulge.
@article{2023MNRAS.521.2745S, author = {{Sanders}, Jason L. and {Matsunaga}, Noriyuki}, title = {{Hunting for C-rich long-period variable stars in the Milky Way's bar-bulge using unsupervised classification of Gaia BP/RP spectra}}, journal = {\mnras}, keywords = {stars: AGB, stars: variables: general, Galaxy: bulge, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2023}, month = may, volume = {521}, number = {2}, pages = {2745-2764}, doi = {10.1093/mnras/stad574}, archiveprefix = {arXiv}, eprint = {2302.10022}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.521.2745S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
A kinematic calibration of the O-rich Mira variable period-age relation from Gaia

Hanyuan Zhang, and Jason L. Sanders

MNRAS, May 2023

Abs ADS arXiv pdf Bib DOI

Empirical and theoretical studies have demonstrated that the periods of Mira variable stars are related to their ages. This, together with their brightness in the infrared, makes them powerful probes of the formation and evolution of highly-extincted or distant parts of the Local Group. Here we utilize the Gaia DR3 catalogue of long-period variable candidates to calibrate the period-age relation of the Mira variables. Dynamical models are fitted to the O-rich Mira variable population across the extended solar neighbourhood and then the resulting solar neighbourhood period-kinematic relations are compared to external calibrations of the age-kinematic relations to derive a Mira variable period-age relation of \(τ≈(6.9\pm 0.3) \mathrmGyr(1+\tanh ((330 \mathrmd-P)/(400\pm 90)\mathrmd)\) . Our results compare well with previous calibrations using smaller data sets as well as the period-age properties of Local Group cluster members. This calibration opens the possibility of accurately characterizing the star formation and the impact of different evolutionary processes throughout the Local Group.
@article{2023MNRAS.521.1462Z, author = {{Zhang}, Hanyuan and {Sanders}, Jason L.}, title = {{A kinematic calibration of the O-rich Mira variable period-age relation from Gaia}}, journal = {\mnras}, keywords = {Galaxy: disc, Galaxy: evolution, Galaxy: kinematics and dynamics, stars: variables: general, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2023}, month = may, volume = {521}, number = {1}, pages = {1462-1478}, doi = {10.1093/mnras/stad575}, archiveprefix = {arXiv}, eprint = {2302.10024}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2023MNRAS.521.1462Z}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2022

MNRAS
Mira variables in the Milky Way’s nuclear stellar disc: discovery and classification

Jason L. Sanders, Noriyuki Matsunaga, Daisuke Kawata, and 3 more authors

MNRAS, Nov 2022

Abs ADS arXiv pdf Bib DOI

The properties of the Milky Way’s nuclear stellar disc give crucial information on the epoch of bar formation. Mira variables are promising bright candidates to study the nuclear stellar disc, and through their period-age relation dissect its star formation history. We report on a sample of 1782 Mira variable candidates across the central \(3\times 3 \mathrmdeg^2\) of the Galaxy using the multi-epoch infrared VISTA Variables in Via Lactea (VVV) survey. We describe the algorithms employed to select candidate variable stars and then model their light curves using periodogram and Gaussian process methods. By combining with WISE, 2MASS, and other archival photometry, we model the multiband light curves to refine the periods and inspect the amplitude variation between different photometric bands. The infrared brightness of the Mira variables means many are too bright and missed by VVV. However, our sample follows a well- defined selection function as expected from artificial star tests. The multiband photometry is modelled using stellar models with circumstellar dust that characterize the mass-loss rates. We demonstrate how \(≳\) 90 per cent of our sample is consistent with O-rich chemistry. Comparison to period-luminosity relations demonstrates that the bulk of the short period stars are situated at the Galactic Centre distance. Many of the longer period variables are very dusty, falling significantly under the O-rich Magellanic Cloud and solar neighbourhood period-luminosity relations and exhibit high mass- loss rates of \(∼2.5\times 10^-5M_⊙ \mathrmyr^-1\) . The period distribution appears consistent with the nuclear stellar disc forming \(rsim 8 \mathrmGyr\) ago, although it is not possible to disentangle the relative contributions of the nuclear stellar disc and the contaminating bulge.
@article{2022MNRAS.517..257S, author = {{Sanders}, Jason L. and {Matsunaga}, Noriyuki and {Kawata}, Daisuke and {Smith}, Leigh C. and {Minniti}, Dante and {Lucas}, Philip W.}, title = {{Mira variables in the Milky Way's nuclear stellar disc: discovery and classification}}, journal = {\mnras}, keywords = {stars: AGB and post-AGB, stars: variables: general, Galaxy: bulge, Galaxy: centre, Galaxy: stellar content, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2022}, month = nov, volume = {517}, number = {1}, pages = {257-280}, doi = {10.1093/mnras/stac2274}, archiveprefix = {arXiv}, eprint = {2208.04966}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.517..257S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
A machine learning approach to photometric metallicities of giant stars

Connor P. Fallows, and Jason L. Sanders

MNRAS, Nov 2022

Abs ADS arXiv pdf Bib DOI

Despite the advances provided by large-scale photometric surveys, stellar features - such as metallicity - generally remain limited to spectroscopic observations often of bright, nearby low-extinction stars. To rectify this, we present a neural network approach for estimating the metallicities and distances of red giant stars with 8-band photometry and parallaxes from Gaia EDR3 and the 2MASS and WISE surveys. The algorithm accounts for uncertainties in the predictions arising from the range of possible outputs at each input and from the range of models compatible with the training set (through drop-out). A two-stage procedure is adopted where an initial network to estimate photoastrometric parallaxes is trained using a large sample of noisy parallax data from Gaia EDR3 and then a secondary network is trained using spectroscopic metallicities from the APOGEE and LAMOST surveys and an augmented feature space utilizing the first-stage parallax estimates. The algorithm produces metallicity predictions with an average uncertainty of \(\pm 0.19 \mathrmdex\) . The methodology is applied to stars within the Galactic bar/bulge with particular focus on a sample of 1.69 million objects with Gaia radial velocities. We demonstrate the use and validity of our approach by inspecting both spatial and kinematic gradients with metallicity in the Galactic bar/bulge recovering previous results on the vertical metallicity gradient (-0.528 \(\pm\) 0.002 dex kpc\(^-1\) ) and the vertex deviation of the bar (\(-21.29\pm 2.74 \mathrmdeg\) ).
@article{2022MNRAS.516.5521F, author = {{Fallows}, Connor P. and {Sanders}, Jason L.}, title = {{A machine learning approach to photometric metallicities of giant stars}}, journal = {\mnras}, keywords = {methods: statistical, stars: distances, Galaxy: abundances, Galaxy: bulge, Galaxy: stellar content, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = nov, volume = {516}, number = {4}, pages = {5521-5537}, doi = {10.1093/mnras/stac2550}, archiveprefix = {arXiv}, eprint = {2209.02553}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.516.5521F}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Information content of BP/RP spectra in Gaia DR3

Callum E. C. Witten, David S. Aguado, Jason L. Sanders, and 6 more authors

MNRAS, Nov 2022

Abs ADS arXiv pdf Bib DOI

Gaia Data Release 3 has provided the astronomical community with the largest stellar spectroscopic survey to date (> 220 million sources). The low resolution (R\(∼\) 50) blue photometer (BP) and red photometer (RP) spectra will allow for the estimation of stellar atmospheric parameters such as effective temperature, surface gravity, and metallicity. We create mock Gaia BP/RP spectra and use Fisher information matrices to probe the resolution limit of stellar parameter measurements using BP/RP spectra. The best-case scenario uncertainties that this analysis provides are then used to produce a mock-observed stellar population in order to evaluate the false positive rate (FPR) of identifying extremely metal-poor stars. We conclude that the community will be able to confidently identify metal-poor stars at magnitudes brighter than G = 16 using BP/RP spectra. At fainter magnitudes true detections will start to be overwhelmed by false positives. When adopting the commonly-used G < 14 limit for metal-poor star searches, we find a FPR for the low- metallicity regimes [Fe/H] < -2, -2.5, and -3 of just 14 \(\ \rm per cent\) , 33 \( \rm per cent\) , and 56 \( \rm per\ cent\) respectively, offering the potential for significant improvements on previous targeting campaigns. Additionally, we explore the chemical sensitivity obtainable directly from BP/RP spectra for carbon and \(α\) -elements. We find an absolute carbon abundance uncertainty of \(σ\) \(_A(C)\) < 1 dex for carbon-enriched metal-poor (CEMP) stars, indicating the potential to identify a CEMP stellar population for follow-up confirmation with higher resolution spectroscopy. Finally, we find that large uncertainties in \(α\) -element abundance measurements using BP/RP spectra means that efficiently obtaining these abundances will be challenging.
@article{2022MNRAS.516.3254W, author = {{Witten}, Callum E.~C. and {Aguado}, David S. and {Sanders}, Jason L. and {Belokurov}, Vasily and {Evans}, N. Wyn and {Koposov}, Sergey E. and {Allende Prieto}, Carlos and {De Angeli}, Francesca and {Irwin}, Mike J.}, title = {{Information content of BP/RP spectra in Gaia DR3}}, journal = {\mnras}, keywords = {stars: abundances, stars: chemically peculiar, stars: fundamental parameters, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = nov, volume = {516}, number = {3}, pages = {3254-3265}, doi = {10.1093/mnras/stac2273}, archiveprefix = {arXiv}, eprint = {2205.12271}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.516.3254W}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJ
Simulated Bars May Be Shorter but Are Not Slower Than Those Observed: TNG50 versus MaNGA

Neige Frankel, Annalisa Pillepich, Hans-Walter Rix, and 6 more authors

ApJ, Nov 2022

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Galactic bars are prominent dynamical structures within disk galaxies whose size, formation time, strength, and pattern speed influence the dynamical evolution of their hosts’ galaxies. Yet, their formation and evolution in a cosmological context is not well understood, as cosmological simulation studies have been limited by the classic trade-off between simulation volume and resolution. Here we analyze barred disk galaxies in the cosmological magnetohydrodynamical simulation TNG50 and quantitatively compare the distributions of bar size and pattern speed to those from MaNGA observations at z = 0. TNG50 galaxies are selected to match the stellar mass and size distributions of observed galaxies, to account for observational selection effects. We find that the high resolution of TNG50 yields bars with a wide range of pattern speeds (including those with \(≥\) 40 km s\(^-1\) kpc\(^-1\) ) and a mean value of \(∼\) 36 km s\(^-1\) kpc larger than those from observations by only 6 km s\(^-1\) kpc\(^-1\) , in contrast with previous lower-resolution cosmological simulations that produced bars that were too slow. We find, however, that the bars in TNG50 are on average \(∼\) 35% shorter than observed, although this discrepancy may partly reflect the remaining inconsistencies in the simulation-data comparison. This leads to higher values of \( \mathcal R \,=\,R_\mathrmcorot/R_\rmb\) in TNG50, but points to simulated bars being too short rather than too slow. After repeating the analysis on the lower-resolution run of the same simulation (with the same physical model), we qualitatively reproduce the results obtained in previous studies: this implies that, along with physical model variations, numerical resolution effects may explain the previously found slowness of simulated bars.
@article{2022ApJ...940...61F, author = {{Frankel}, Neige and {Pillepich}, Annalisa and {Rix}, Hans-Walter and {Rodriguez-Gomez}, Vicente and {Sanders}, Jason and {Bovy}, Jo and {Kollmeier}, Juna and {Murray}, Norm and {Mackereth}, Ted}, title = {{Simulated Bars May Be Shorter but Are Not Slower Than Those Observed: TNG50 versus MaNGA}}, journal = {\apj}, keywords = {Barred spiral galaxies, Disk galaxies, Galaxies, 136, 391, 573, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = nov, volume = {940}, number = {1}, eid = {61}, pages = {61}, doi = {10.3847/1538-4357/ac9972}, archiveprefix = {arXiv}, eprint = {2201.08406}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022ApJ...940...61F}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The extinction law in the inner 3 \texttimes 3 deg\(^2\) of the Milky Way and the red clump absolute magnitude in the inner bar-bulge

Jason L. Sanders, Leigh Smith, Carlos González-Fernández, and 2 more authors

MNRAS, Aug 2022

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The extinction law from 0.9 to 8 microns in the inner \(3\times 3\deg ^2\) of the Milky Way is measured using data from VISTA Variables in the Via Lactea, GLIMPSE, and WISE. Absolute extinction ratios are found by requiring that the observed red clump density peaks at the GRAVITY collaboration distance to the Galactic centre. When combined with selective extinction ratios measured from the bulge giant colour-colour diagrams, we find an extinction law of \(A_Z:A_Y:A_J:A_H:A_K_s:A_W1:A_[3.6]:A_[4.5]:A_W2:A_[5 .8]:A_[8.0] =7.19(0.30):5.11(0.20):3.23(0.11):1.77(0.04):1:0. 54(0.02):0.46(0.03):0.34(0.03):0.32(0.03):0.24(0.04):0.28(0.03)\) valid for low extinctions where non-linearities are unimportant. These results imply an extinction law from the Rayleigh Jeans colour excess method of \(A_K_s=0.677(H-[4.5]-0.188)\) . We find little evidence for significant selective extinction ratio variation over the inspected region (around \(5 \mathrmper\,cent\) ). Assuming the absolute extinction ratios do not vary across the inspected region gives an independent measurement of the absolute K\(_s\) magnitude of the red clump at the Galactic Centre of \((-1.61\pm 0.07) \mathrmmag\) . This is very similar to the value measured for solar neighbourhood red clump stars giving confidence in the use of red clump stars as standard candles across the Galaxy. As part of our analysis, we inspect the completeness of PSF photometry from the VVV survey using artificial star tests, finding \(90 \mathrmper\,cent\) completeness at \(K_s≈16  (17)\) in high (low) density regions and good agreement with the number counts with respect to the GALACTICNUCLEUS and DECAPS catalogues over small regions of the survey.
@article{2022MNRAS.514.2407S, author = {{Sanders}, Jason L. and {Smith}, Leigh and {Gonz{\'a}lez-Fern{\'a}ndez}, Carlos and {Lucas}, Philip and {Minniti}, Dante}, title = {{The extinction law in the inner 3 {\texttimes} 3 deg\(^{2}\)\ of the Milky Way and the red clump absolute magnitude in the inner bar-bulge}}, journal = {\mnras}, keywords = {stars: distances, dust, extinction, Galaxy: bulge, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = aug, volume = {514}, number = {2}, pages = {2407-2424}, doi = {10.1093/mnras/stac1367}, archiveprefix = {arXiv}, eprint = {2205.10378}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.514.2407S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
The history of the inner Milky Way: Variable stars across the Galactic Bulge and Nuclear Stellar Disc

Jason Sanders

In Inward Bound: Bulges from High Redshifts to the Milky Way, May 2022

Abs ADS Bib DOI

Variable stars are precision tools for decoding the formation and evolution of the Milky Way’s bar/bulge. Not only do variable stars often follow tight period-luminosity relations enabling accurate 3D mapping, but their pulsations are often related to intrinsic properties such as age and metallicity. For instance, the asymmetry of RR Lyrae light curves is often related to their metallicity, whilst the periods of Mira variable stars are believed to be related to their age. From this information we can begin to disentangle the sequence of events that formed the Milky Way’s bar/bulge. \\textbackslashI describe searches for variable stars across the Galactic bulge and the nuclear stellar disc using the VISTA Variables in the Via Lactea (VVV) survey. The first of these is a targeted search for Mira variable stars within the nuclear stellar disc. \(∼\) 1800 Mira variables have been discovered from which variation of proper motion dispersion with period suggests the bar formed \(∼\) 8-10 Gyr ago. The second approach is a supervised classification of 490 million light curves from which \(∼\) 40000 high- confidence RR Lyrae ab have been found. I will describe ongoing efforts to model the joint kinematic-metallicity distribution of this population for deciphering the early formation history of the bar-bulge.
@inproceedings{2022bulg.confE..14S, author = {{Sanders}, Jason}, title = {{The history of the inner Milky Way: Variable stars across the Galactic Bulge and Nuclear Stellar Disc}}, keywords = {Bulges, Zenodo community bulges2022}, booktitle = {Inward Bound: Bulges from High Redshifts to the Milky Way}, year = {2022}, month = may, pages = {14}, doi = {10.5281/zenodo.6545482}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022bulg.confE..14S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Self-consistent modelling of the Milky Way’s nuclear stellar disc

Mattia C. Sormani, Jason L. Sanders, Tobias K. Fritz, and 14 more authors

MNRAS, May 2022

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The nuclear stellar disc (NSD) is a flattened high-density stellar structure that dominates the gravitational field of the Milky Way at Galactocentric radius \(30 \rm pc≲R≲300 \rm pc\) . We construct axisymmetric self-consistent equilibrium dynamical models of the NSD in which the distribution function is an analytic function of the action variables. We fit the models to the normalized kinematic distributions (line-of-sight velocities + VIRAC2 proper motions) of stars in the NSD survey of Fritz et al., taking the foreground contamination due to the Galactic Bar explicitly into account using an N-body model. The posterior marginalized probability distributions give a total mass of \(M_\rm NSD = 10.5^+1.1_-1.0 \times 10^8   \rm M_⊙\) , roughly exponential radial and vertical scale lengths of \(R_\rm disc = 88.6^+9.2_-6.9  \rm pc\) and \(H_\rm disc=28.4^+5.5_-5.5  \rm pc\) , respectively, and a velocity dispersion \(σ≃70 \rm km\,s^-1\) that decreases with radius. We find that the assumption that the NSD is axisymmetric provides a good representation of the data. We quantify contamination from the Galactic Bar in the sample, which is substantial in most observed fields. Our models provide the full 6D (position + velocity) distribution function of the NSD, which can be used to generate predictions for future surveys. We make the models publicly available as part of the software package AGAMA.
@article{2022MNRAS.512.1857S, author = {{Sormani}, Mattia C. and {Sanders}, Jason L. and {Fritz}, Tobias K. and {Smith}, Leigh C. and {Gerhard}, Ortwin and {Sch{\"o}del}, Rainer and {Magorrian}, John and {Neumayer}, Nadine and {Nogueras-Lara}, Francisco and {Feldmeier-Krause}, Anja and {Mastrobuono-Battisti}, Alessandra and {Schultheis}, Mathias and {Shahzamanian}, Banafsheh and {Vasiliev}, Eugene and {Klessen}, Ralf S. and {Lucas}, Philip and {Minniti}, Dante}, title = {{Self-consistent modelling of the Milky Way's nuclear stellar disc}}, journal = {\mnras}, keywords = {Galaxy: centre, Galaxy: kinematics and dynamics, Galaxy: structure, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = may, volume = {512}, number = {2}, pages = {1857-1884}, doi = {10.1093/mnras/stac639}, archiveprefix = {arXiv}, eprint = {2111.12713}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.512.1857S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Variable star classification across the Galactic bulge and disc with the VISTA Variables in the Vı́a Láctea survey

Thomas A. Molnar, Jason L. Sanders, Leigh C. Smith, and 3 more authors

MNRAS, Jan 2022

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We present VIVACE, the VIrac VAriable Classification Ensemble, a catalogue of variable stars extracted from an automated classification pipeline for the Vista Variables in the Vı́a Láctea (VVV) infrared survey of the Galactic bar/bulge and southern disc. Our procedure utilizes a two-stage hierarchical classifier to first isolate likely variable sources using simple variability summary statistics and training sets of non-variable sources from the Gaia early third data release, and then classify candidate variables using more detailed light-curve statistics and training labels primarily from OGLE and VSX. The methodology is applied to point-spread-function photometry for \(∼\) 490 million light curves from the VIRAC v2 astrometric and photometric catalogue resulting in a catalogue of \(∼\) 1.4 million likely variable stars, of which \(∼\) 39 000 are high-confidence (classification probability >0.9) RR Lyrae ab stars, \(∼\) 8000 RR Lyrae c/d stars, \(∼\) 187, 000 detached/semi-detached eclipsing binaries, \(∼\) 18, 000 contact eclipsing binaries, \(∼\) 1400 classical Cepheid variables and \(∼\) 2200 Type II Cepheid variables. Comparison with OGLE-4 suggests a completeness of around \(90  \mathrmper\,cent\) for RRab and \(≲60 \mathrmper\,cent\) for RRc/d, and a misclassification rate for known RR Lyrae stars of around \(1 \mathrmper\,cent\) for the high confidence sample. We close with two science demonstrations of our new VIVACE catalogue: first, a brief investigation of the spatial and kinematic properties of the RR Lyrae stars within the disc/bulge, demonstrating the spatial elongation of bar- bulge RR Lyrae stars is in the same sense as the more metal-rich red giant population whilst having a slower rotation rate of \(∼\!40 \mathrmkm\,s^-1\mathrmkpc^-1\) ; and secondly, an investigation of the GaiaEDR3 parallax zero-point using contact eclipsing binaries across the Galactic disc plane and bulge.
@article{2022MNRAS.509.2566M, author = {{Molnar}, Thomas A. and {Sanders}, Jason L. and {Smith}, Leigh C. and {Belokurov}, Vasily and {Lucas}, Philip and {Minniti}, Dante}, title = {{Variable star classification across the Galactic bulge and disc with the VISTA Variables in the V{\'\i}a L{\'a}ctea survey}}, journal = {\mnras}, keywords = {catalogues, surveys, binaries: eclipsing, stars: variables: general, stars: variables: RR Lyrae, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2022}, month = jan, volume = {509}, number = {2}, pages = {2566-2592}, doi = {10.1093/mnras/stab3116}, archiveprefix = {arXiv}, eprint = {2110.15371}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2022MNRAS.509.2566M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2021

MNRAS
Evidence for sub-Chandrasekhar Type Ia supernovae from the last major merger

Jason L. Sanders, Vasily Belokurov, and Kai T. F. Man

MNRAS, Sep 2021

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We investigate the contribution of sub-Chandrasekhar mass Type Ia supernovae to the chemical enrichment of the Gaia Sausage galaxy, the progenitor of a significant merger event in the early life of the Milky Way. Using a combination of data from Nissen & Schuster, the GALactic Archaeology with HERMES (GALAH) Data Release 3 [with 1D non-local thermal equilibrium (NLTE) abundance corrections], and the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Data Release 16, we fit analytic chemical evolution models to a nine-dimensional chemical abundance space (Fe, Mg, Si, Ca, Cr, Mn, Ni, Cu, and Zn) in particular focusing on the iron-peak elements, Mn and Ni. We find that low [Mn/Fe] \(∼-0.15 \mathrmdex\) and low [Ni/Fe] \(∼-0.3 \mathrmdex\) Type Ia yields are required to explain the observed trends beyond the [\(α\) /Fe] knee of the Gaia Sausage (approximately at [Fe/H] \(=-1.4 \mathrmdex\) ). Comparison to theoretical yield calculations indicates a significant contribution from sub-Chandrasekhar mass Type Ia supernovae in this system (from \(∼ 60 \mathrmper\,cent\) to \(100 \mathrmper\,cent\) depending on the theoretical model with an additional \(\pm 10 \mathrmper\,cent\) systematic from NLTE corrections). We compare to results from other Local Group environments including dwarf spheroidal galaxies, the Magellanic Clouds, and the Milky Way’s bulge, finding the Type Ia [Mn/Fe] yield must be metallicity dependent. Our results suggest that sub-Chandrasekhar mass channels are a significant, perhaps even dominant, contribution to Type Ia supernovae in metal-poor systems, whilst more metal-rich systems could be explained by metallicity-dependent sub-Chandrasekhar mass yields, possibly with additional progenitor mass variation related to star formation history, or an increased contribution from Chandrasekhar mass channels at higher metallicity.
@article{2021MNRAS.506.4321S, author = {{Sanders}, Jason L. and {Belokurov}, Vasily and {Man}, Kai T.~F.}, title = {{Evidence for sub-Chandrasekhar Type Ia supernovae from the last major merger}}, journal = {\mnras}, keywords = {nuclear reactions, nucleosynthesis, abundances, supernovae: general, Galaxy: abundances, Galaxy: evolution, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2021}, month = sep, volume = {506}, number = {3}, pages = {4321-4343}, doi = {10.1093/mnras/stab1951}, archiveprefix = {arXiv}, eprint = {2106.11324}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2021MNRAS.506.4321S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
VVV-WIT-08: the giant star that blinked

Leigh C. Smith, Sergey E. Koposov, Philip W. Lucas, and 11 more authors

MNRAS, Aug 2021

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We report the serendipitous discovery of a late-type giant star that exhibited a smooth, eclipse-like drop in flux to a depth of 97 per cent. Minimum flux occurred in 2012 April and the total event duration was a few hundred days. Light curves in V, I, and K\(_s\) from the Optical Gravitational Lensing Experiment and VISTA Variables in the Via Lactea surveys show a remarkably achromatic event. During 17 yr of observational coverage of this source only one such event was detected. The physical properties of the giant star itself appear somewhat unusual, which may ultimately provide a clue towards the nature of the system. By modelling the event as an occultation by an object that is elliptical in projection with uniform transparency, we place limits on its physical size and velocity. We find that the occultation is unlikely to be due to a chance alignment with a foreground object. We consider a number of possible candidates for the occulter, which must be optically thick and possess a radius or thickness in excess of 0.25 au. None are completely satisfactory matches to all the data. The duration, depth, and relative achromaticity of the dip mark this out as an exceptionally unusual event, whose secret has still not been fully revealed. We find two further candidates in the VVV survey and we suggest that these systems, and two previously known examples, may point to a broad class of long-period eclipsing binaries wherein a giant star is occulted by a circumsecondary disc.
@article{2021MNRAS.505.1992S, author = {{Smith}, Leigh C. and {Koposov}, Sergey E. and {Lucas}, Philip W. and {Sanders}, Jason L. and {Minniti}, Dante and {Udalski}, Andrzej and {Evans}, N. Wyn and {Aguado}, David and {Ivanov}, Valentin D. and {Saito}, Roberto K. and {Fraga}, Luciano and {Pietrukowicz}, Pawel and {Penoyre}, Zephyr and {Gonz{\'a}lez-Fern{\'a}ndez}, Carlos}, title = {{VVV-WIT-08: the giant star that blinked}}, journal = {\mnras}, keywords = {binaries: eclipsing, stars: individual: epsilon Aur, stars: individual: TYC 2505-672-1, stars: individual: ASASSN-21co, stars: peculiar, Astrophysics - Solar and Stellar Astrophysics}, year = {2021}, month = aug, volume = {505}, number = {2}, pages = {1992-2008}, doi = {10.1093/mnras/stab1211}, archiveprefix = {arXiv}, eprint = {2106.05300}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2021MNRAS.505.1992S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2020

MNRAS
Near-Gaussian distributions for modelling discrete stellar velocity data with heteroskedastic uncertainties

Jason L. Sanders, and N. Wyn Evans

MNRAS, Dec 2020

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The velocity distributions of stellar tracers in general exhibit weak non-Gaussianity encoding information on the orbital composition of a galaxy and the underlying potential. The standard solution for measuring non-Gaussianity involves constructing a series expansion (e.g. the Gauss-Hermite series) that can produce regions of negative probability density. This is a significant issue for the modelling of discrete data with heteroskedastic uncertainties. Here, we introduce a method to construct positive-definite probability distributions by the convolution of a given kernel with a Gaussian distribution. Further convolutions by observational uncertainties are trivial. The statistics (moments and cumulants) of the resulting distributions are governed by the kernel distribution. Two kernels (uniform and Laplace) offer simple drop-in replacements for a Gauss-Hermite series for negative and positive excess kurtosis distributions with the option of skewness. We demonstrate the power of our method by an application to real and mock line-of-sight velocity data sets on dwarf spheroidal galaxies, where kurtosis is indicative of orbital anisotropy and hence a route to breaking the mass-anisotropy degeneracy for the identification of cusped versus cored dark matter profiles. Data on the Fornax dwarf spheroidal galaxy indicate positive excess kurtosis and hence favour a cored dark matter profile. Although designed for discrete data, the analytic Fourier transforms of the new models also make them appropriate for spectral fitting, which could improve the fits of high-quality data by avoiding unphysical negative wings in the line-of-sight velocity distribution.
@article{2020MNRAS.499.5806S, author = {{Sanders}, Jason L. and {Evans}, N. Wyn}, title = {{Near-Gaussian distributions for modelling discrete stellar velocity data with heteroskedastic uncertainties}}, journal = {\mnras}, keywords = {methods: statistical, techniques: radial velocities, techniques: spectroscopic, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2020}, month = dec, volume = {499}, number = {4}, pages = {5806-5825}, doi = {10.1093/mnras/staa2860}, archiveprefix = {arXiv}, eprint = {2009.07858}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2020MNRAS.499.5806S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Models of distorted and evolving dark matter haloes

Jason L. Sanders, Edward J. Lilley, Eugene Vasiliev, and 2 more authors

MNRAS, Dec 2020

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We investigate the ability of basis function expansions to reproduce the evolution of a Milky Way-like dark matter halo, extracted from a cosmological zoom-in simulation. For each snapshot, the density of the halo is reduced to a basis function expansion, with interpolation used to recreate the evolution between snapshots. The angular variation of the halo density is described by spherical harmonics, and the radial variation either by biorthonormal basis functions adapted to handle truncated haloes or by splines. High fidelity orbit reconstructions are attainable using either method with similar computational expense. We quantify how the error in the reconstructed orbits varies with expansion order and snapshot spacing. Despite the many possible biorthonormal expansions, it is hard to beat a conventional Hernquist-Ostriker expansion with a moderate number of terms (\(≳\) 15 radial and \(≳\) 6 angular). As two applications of the developed machinery, we assess the impact of the time-dependence of the potential on (i) the orbits of Milky Way satellites and (ii) planes of satellites as observed in the Milky Way and other nearby galaxies. Time evolution over the last 5 Gyr introduces an uncertainty in the Milky Way satellites’ orbital parameters of \(∼15  \mathrmper\,cent\) , comparable to that induced by the observational errors or the uncertainty in the present- day Milky Way potential. On average, planes of satellites grow at similar rates in evolving and time-independent potentials. There can be more, or less, growth in the plane’s thickness, if the plane becomes less, or more, aligned with the major or minor axis of the evolving halo.
@article{2020MNRAS.499.4793S, author = {{Sanders}, Jason L. and {Lilley}, Edward J. and {Vasiliev}, Eugene and {Evans}, N. Wyn and {Erkal}, Denis}, title = {{Models of distorted and evolving dark matter haloes}}, journal = {\mnras}, keywords = {methods: numerical, galaxies: haloes, galaxies: kinematics and dynamics, galaxies: structure, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2020}, month = dec, volume = {499}, number = {4}, pages = {4793-4813}, doi = {10.1093/mnras/staa3079}, archiveprefix = {arXiv}, eprint = {2009.00645}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2020MNRAS.499.4793S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Fluctuations in galactic bar parameters due to bar-spiral interaction

T. Hilmi, I. Minchev, T. Buck, and 10 more authors

MNRAS, Sep 2020

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We study the late-time evolution of the central regions of two Milky Way (MW)-like simulations of galaxies formed in a cosmological context, one hosting a fast bar and the other a slow one. We find that bar length, R\(_b\) , measurements fluctuate on a dynamical time-scale by up to 100 per cent, depending on the spiral structure strength and measurement threshold. The bar amplitude oscillates by about 15 per cent, correlating with R\(_b\) . The Tremaine-Weinberg method estimates of the bars’ instantaneous pattern speeds show variations around the mean of up to \(∼\!20 \rm per cent\) , typically anticorrelating with the bar length and strength. Through power spectrum analyses, we establish that these bar pulsations, with a period in the range \(∼\) 60-200 Myr, result from its interaction with multiple spiral modes, which are coupled with the bar. Because of the presence of odd spiral modes, the two bar halves typically do not connect at exactly the same time to a spiral arm, and their individual lengths can be significantly offset. We estimated that in about 50 per cent of bar measurements in MW-mass external galaxies, the bar lengths of SBab-type galaxies are overestimated by \(∼\!15 \rm per\ cent\) and those of SBbc types by \(∼\!55 \rm per\ cent\) . Consequently, bars longer than their corotation radius reported in the literature, dubbed ’ultrafast bars’, may simply correspond to the largest biases. Given that the Scutum- Centaurus arm is likely connected to the near half of the MW bar, recent direct measurements may be overestimating its length by 1-1.5 kpc, while its present pattern speed may be 5-10 \(\rm km s^-1 kpc^-1\) smaller than its time-averaged value.
@article{2020MNRAS.497..933H, author = {{Hilmi}, T. and {Minchev}, I. and {Buck}, T. and {Martig}, M. and {Quillen}, A.~C. and {Monari}, G. and {Famaey}, B. and {de Jong}, R.~S. and {Laporte}, C.~F.~P. and {Read}, J. and {Sanders}, J.~L. and {Steinmetz}, M. and {Wegg}, C.}, title = {{Fluctuations in galactic bar parameters due to bar-spiral interaction}}, journal = {\mnras}, keywords = {Galaxy: bulge, Galaxy: fundamental parameters, Galaxy: kinematics and dynamics, galaxies: bar, galaxies: evolution, Astrophysics - Astrophysics of Galaxies}, year = {2020}, month = sep, volume = {497}, number = {1}, pages = {933-955}, doi = {10.1093/mnras/staa1934}, archiveprefix = {arXiv}, eprint = {2003.05457}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2020MNRAS.497..933H}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJ
Keeping It Cool: Much Orbit Migration, yet Little Heating, in the Galactic Disk

Neige Frankel, Jason Sanders, Yuan-Sen Ting, and 1 more author

ApJ, Jun 2020

Abs ADS arXiv pdf Bib DOI

A star in the Milky Way’s disk can now be at a Galactocentric radius quite distant from its birth radius for two reasons: either its orbit has become eccentric through radial heating, which increases its radial action J\(_R\) (“blurring”), or merely its angular momentum L\(_z\) has changed and thereby its guiding radius (“churning”). We know that radial orbit migration is strong in the Galactic low-\(α\) disk and set out to quantify the relative importance of these two effects, by devising and applying a parameterized model ( \(\boldsymbolp_\boldsymbolm\) ) for the distribution \(p(L_z,J_R,τ,\left[\mathrmFe/\rmH\right]| \boldsymbolp_\boldsymbolm)\) in the stellar disk. This model describes the orbit evolution for stars of age \(τ\) and metallicity \(\left[\mathrmFe/\rmH\right]\) , presuming that coeval stars were initially born on (near-)circular orbits, and with a unique \(\left[\mathrmFe/\rmH\right]\) at a given birth angular momentum and age. We fit this model to APOGEE red clump stars, accounting for the complex selection function of the survey. The best-fit model implies changes of angular momentum of \(\sqrt⟨\rm∆L_z⟩^2≈619\,\mathrmkpc\,\mathrmkm\,\rms^-1 (τ/6\mathrmGyr)^0.5\) and changes of radial action as \(\sqrt⟨\rm∆J_R⟩^2≈63\,\mathrmkpc\,\mathrmkm\,\rms^-1(τ/6\mathrmGyr)^0.6\) at 8 kpc. This suggests that the secular orbit evolution of the disk is dominated by diffusion in angular momentum, with radial heating being an order of magnitude lower.
@article{2020ApJ...896...15F, author = {{Frankel}, Neige and {Sanders}, Jason and {Ting}, Yuan-Sen and {Rix}, Hans-Walter}, title = {{Keeping It Cool: Much Orbit Migration, yet Little Heating, in the Galactic Disk}}, journal = {\apj}, keywords = {Galaxy abundances, Galaxy stellar disks, Milky Way Galaxy, Milky Way disk, Milky Way evolution, Milky Way dynamics, Galaxy dynamics, 574, 1594, 1054, 1050, 1052, 1051, 591, Astrophysics - Astrophysics of Galaxies}, year = {2020}, month = jun, volume = {896}, number = {1}, eid = {15}, pages = {15}, doi = {10.3847/1538-4357/ab910c}, archiveprefix = {arXiv}, eprint = {2002.04622}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2020ApJ...896...15F}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The biggest splash

Vasily Belokurov, Jason L. Sanders, Azadeh Fattahi, and 4 more authors

MNRAS, May 2020

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Using a large sample of bright nearby stars with accurate Gaia Data Release 2 astrometry and auxiliary spectroscopy we map out the properties of the principle Galactic components such as the ’thin’ and ’thick’ discs and the halo. We confirm previous claims that in the Solar neighbourhood, there exists a large population of metal-rich ([Fe/H] > -0.7) stars on highly eccentric orbits. By studying the evolution of elemental abundances, kinematics, and stellar ages in the plane of azimuthal velocity V\(_\(φ\) \) and metallicity [Fe/H], we demonstrate that this metal-rich halo-like component, which we dub the Splash, is linked to the \(α\) -rich (or ’thick’) disc. Splash stars have little to no angular momentum and many are on retrograde orbits. They are predominantly old, but not as old as the stars deposited into the Milky Way (MW) in the last major merger. We argue, in agreement with several recent studies, that the Splash stars may have been born in the MW’s protodisc prior to the massive ancient accretion event which drastically altered their orbits. We cannot, however, rule out other (alternative) formation channels. Taking advantage of the causal connection between the merger and the Splash, we put constraints of the epoch of the last massive accretion event to have finished 9.5 Gyr ago. The link between the local metal-rich and metal-poor retrograde stars is confirmed using a large suite of cutting- edge numerical simulations of the MW’s formation.
@article{2020MNRAS.494.3880B, author = {{Belokurov}, Vasily and {Sanders}, Jason L. and {Fattahi}, Azadeh and {Smith}, Martin C. and {Deason}, Alis J. and {Evans}, N. Wyn and {Grand}, Robert J.~J.}, title = {{The biggest splash}}, journal = {\mnras}, keywords = {Galaxy: formation, Galaxy: halo, galaxies: dwarf, Local Group, Astrophysics - Astrophysics of Galaxies}, year = {2020}, month = may, volume = {494}, number = {3}, pages = {3880-3898}, doi = {10.1093/mnras/staa876}, archiveprefix = {arXiv}, eprint = {1909.04679}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2020MNRAS.494.3880B}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Models of bars - II. Exponential profiles

D. P. McGough, N. W. Evans, and J. L. Sanders

MNRAS, Apr 2020

Abs ADS arXiv pdf Bib DOI

We present a new model for galactic bars with exponentially falling major axis luminosity profiles and Gaussian cross-sections. This is based on the linear superposition of Gaussian potential- density pairs with an exponential weight function, using an extension of the method originally introduced by Long & Murali. We compute the density, potential, and forces, using Gaussian quadrature. These quantities are given as explicit functions of position. There are three independent scaled bar parameters that can be varied continuously to produce bespoke bars of a given mass and shape. We categorize the effective potential by splitting a reduced parameter space into six regions. Unusually, we find bars with three stable Lagrange points on the major axis are possible. Our model reveals a variety of unexpected orbital structure, including a bifurcating x\(_1\) orbit coexisting with a stable x\(_4\) orbit. Propeller orbits are found to play a dominant role in the orbital structure, and we find striking similarities between our bar configuration and the model of Kaufmann & Contopoulos. We find a candidate orbital family, sired from the propeller orbits, that may be responsible for the observed high-velocity peaks in the Milky Way’s bar. As a cross- check, we inspect, for the first time, the proper motions of stars in the high-velocity peaks, which also match our suggested orbital family well. This work adds to the increasing body of evidence that real galactic bars may be supported at least partly by propeller orbits rather than solely by elliptical-like orbits of the x\(_1\) family.
@article{2020MNRAS.493.2676M, author = {{McGough}, D.~P. and {Evans}, N.~W. and {Sanders}, J.~L.}, title = {{Models of bars - II. Exponential profiles}}, journal = {\mnras}, keywords = {Galaxy: bulge, galaxies: kinematics and dynamics, galaxies: structure, Astrophysics - Astrophysics of Galaxies}, year = {2020}, month = apr, volume = {493}, number = {2}, pages = {2676-2687}, doi = {10.1093/mnras/staa491}, archiveprefix = {arXiv}, eprint = {1912.02834}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2020MNRAS.493.2676M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

Transverse bar/bulge kinematics with Gaia and VVV

Jason L. Sanders, N. Wyn Evans, Leigh Smith, and 1 more author

In Galactic Dynamics in the Era of Large Surveys, Jan 2020

Abs ADS Bib DOI

We present new results on the Galactic bar/bulge transverse velocity structure using Gaia and the VISTA Variables in Via Lactea (VVV) survey. Gaia is complemented in high extinction regions by the multi-epoch infrared VVV observations for which derived relative proper motions can be tied to Gaia’s absolute frame. We extract kinematic maps (both 2D and 3D) of the Galactic bar/bulge, from which we measure the pattern speed of the bar using a novel technique. We focus on the evidence of an X-shaped bulge from the kinematic maps.

@inproceedings{2020IAUS..353...38S,
  author = {{Sanders}, Jason L. and {Evans}, N. Wyn and {Smith}, Leigh and {Lucas}, Philip},
  title = {{Transverse bar/bulge kinematics with Gaia and VVV}},
  keywords = {Galaxy: structure, bulge, kinematics and dynamics, fundamental parameters},
  booktitle = {Galactic Dynamics in the Era of Large Surveys},
  year = {2020},
  editor = {{Valluri}, Monica and {Sellwood}, J.~A.},
  series = {IAU Symposium},
  volume = {353},
  month = jan,
  pages = {38-42},
  doi = {10.1017/S1743921319008664},
  adsurl = {https://ui.adsabs.harvard.edu/abs/2020IAUS..353...38S},
  adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

2019

MNRAS
The total stellar halo mass of the Milky Way

Alis J. Deason, Vasily Belokurov, and Jason L. Sanders

MNRAS, Dec 2019

Abs ADS arXiv pdf Bib DOI

We measure the total stellar halo luminosity using red giant branch (RGB) stars selected from Gaia data release 2. Using slices in magnitude, colour, and location on the sky, we decompose RGB stars belonging to the disc and halo by fitting two-dimensional Gaussians to the Galactic proper motion distributions. The number counts of RGB stars are converted to total stellar halo luminosity using a suite of isochrones weighted by age and metallicity, and by applying a volume correction based on the stellar halo density profile. Our method is tested and calibrated using Galaxia and N-body models. We find a total luminosity (out to 100 kpc) of L_halo = 7.9 \(\pm\) 2.0 \texttimes 10\^8 L_\(⊙\) excluding Sgr, and L_halo = 9.4 \(\pm\) 2.4 \texttimes 10\^8 L_\(⊙\) including Sgr. These values are appropriate for our adopted stellar halo density profile and metallicity distribution, but additional systematics related to these assumptions are quantified and discussed. Assuming a stellar mass-to-light ratio appropriate for a Kroupa initial mass function (M\(^\(⋆\) \) /L = 1.5), we estimate a stellar halo mass of M\^\textbackslashstar _halo = 1.4 \(\pm\) 0.4\texttimes 10\^9 M_\(⊙\) . This mass is larger than previous estimates in the literature, but is in good agreement with the emerging picture that the (inner) stellar halo is dominated by one massive dwarf progenitor. Finally, we argue that the combination of a {\(∼\) }10\^9 M_\(⊙\) mass and an average metallicity of <[Fe/H]> \(∼\) -1.5 for the Galactic halo points to an ancient (\(∼\) 10 Gyr) merger event.
@article{2019MNRAS.490.3426D, author = {{Deason}, Alis J. and {Belokurov}, Vasily and {Sanders}, Jason L.}, title = {{The total stellar halo mass of the Milky Way}}, journal = {\mnras}, keywords = {Galaxy: halo, Galaxy: kinematics and dynamics, Galaxy: stellar content, Astrophysics - Astrophysics of Galaxies}, year = {2019}, month = dec, volume = {490}, number = {3}, pages = {3426-3439}, doi = {10.1093/mnras/stz2793}, archiveprefix = {arXiv}, eprint = {1908.02763}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.490.3426D}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The pattern speed of the Milky Way bar from transverse velocities

Jason L. Sanders, Leigh Smith, and N. Wyn Evans

MNRAS, Oct 2019

Abs ADS arXiv pdf Bib DOI

We use the continuity equation to derive a method for measuring the pattern speed of the Milky Way’s bar/bulge from proper motion data. The method has minimal assumptions but requires complete coverage of the non-axisymmetric component in two of the three Galactic coordinates. We apply our method to the proper motion data from a combination of Gaia DR2 and VISTA Variables in the Via Lactea (VVV) to measure the pattern speed of the bar as \(Ω\) _p=(41\(\pm\) 3) km s\^{-1 kpc\^{-1}} (where the error is statistical). This puts the corotation radius at (5.7\(\pm\) 0.4) kpc, under the assumptions of the standard peculiar motion of the Sun and the absence of non-axisymmetric streaming in the Solar neighbourhood. The obtained result uses only data on the near side of the bar which produces consistent measurements of the distance and velocity of the centre of the Galaxy. Addition of the data on the far side of the bar pulls the pattern speed down to \(Ω\) _p=(31\(\pm\) 1) km s\^{-1 kpc\^{-1}} but requires a lower transverse velocity for the Galactic centre than observed. This suggests systematics of 5-10 km s\^{-1kpc\^{-1}} dominate the uncertainty. We demonstrate using a dynamically formed bar/bulge simulation that even with the limited field of view of the VVV survey our method robustly recovers the pattern speed.
@article{2019MNRAS.488.4552S, author = {{Sanders}, Jason L. and {Smith}, Leigh and {Evans}, N. Wyn}, title = {{The pattern speed of the Milky Way bar from transverse velocities}}, journal = {\mnras}, keywords = {Galaxy: bulge, Galaxy: fundamental parameters, Galaxy: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2019}, month = oct, volume = {488}, number = {4}, pages = {4552-4564}, doi = {10.1093/mnras/stz1827}, archiveprefix = {arXiv}, eprint = {1903.02009}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.488.4552S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJ
The Inside-out Growth of the Galactic Disk

Neige Frankel, Jason Sanders, Hans-Walter Rix, and 2 more authors

ApJ, Oct 2019

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We quantify the inside-out growth of the Milky Way’s low-\(α\) stellar disk, modeling the ages, metallicities, and Galactocentric radii of APOGEE red clump stars with 6 kpc < R < 13 kpc. The current stellar distribution differs significantly from that expected from the star formation history due to the redistribution of stars through radial orbit mixing. We propose and fit a global model for the Milky Way disk, specified by an inside-out star formation history, radial orbit mixing, and an empirical, parametric model for its chemical evolution. We account for the spatially complex survey selection function, and find that the model fits all data well. We find distinct inside-out growth of the Milky Way disk; the best-fit model implies that the half-mass radius of the Milky Way disk has grown by 43% over the last 7 Gyr. Yet, such inside-out growth still results in a present-day age gradient weaker than 0.1 Gyr kpc\(^-1\) . Our model predicts the half-mass and half-light sizes of the Galactic disk at earlier epochs, which can be compared to the observed redshift-size relations of disk galaxies. We show that radial orbit migration can reconcile the distinct disk-size evolution with redshift, also expected from cosmological simulations, with the modest present-day age gradients seen in the Milky Way and other galaxies.
@article{2019ApJ...884...99F, author = {{Frankel}, Neige and {Sanders}, Jason and {Rix}, Hans-Walter and {Ting}, Yuan-Sen and {Ness}, Melissa}, title = {{The Inside-out Growth of the Galactic Disk}}, journal = {\apj}, keywords = {Galaxy formation, Milky Way disk, Milky Way formation, 595, 1050, 1053, Astrophysics - Astrophysics of Galaxies}, year = {2019}, month = oct, volume = {884}, number = {2}, eid = {99}, pages = {99}, doi = {10.3847/1538-4357/ab4254}, archiveprefix = {arXiv}, eprint = {1909.07118}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019ApJ...884...99F}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2

G. Torrealba, V. Belokurov, S. E. Koposov, and 8 more authors

MNRAS, Sep 2019

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We report the discovery of a Milky Way satellite in the constellation of Antlia. The Antlia 2 dwarf galaxy is located behind the Galactic disc at a latitude of b \(∼\) 11\textdegree and spans 1.26\textdegree, which corresponds to \(∼\) 2.9 kpc at its distance of 130 kpc. While similar in spatial extent to the Large Magellanic Cloud, Antlia 2 is orders of magnitude fainter at M\(_V\) = -9 mag, making it by far the lowest surface brightness system known (at \(∼\) 31.9 mag arcsec\(^-2\) ), \(∼\) 100 times more diffuse than the so-called ultra diffuse galaxies. The satellite was identified using a combination of astrometry, photometry, and variability data from Gaia Data Release 2, and its nature confirmed with deep archival DECam imaging, which revealed a conspicuous BHB signal. We have also obtained follow-up spectroscopy using AAOmega on the AAT, identifying 159 member stars, and we used them to measure the dwarf’s systemic velocity, 290.9 \(\pm\) 0.5 km s\(^-1\) , its velocity dispersion, 5.7 \(\pm\) 1.1 km s\(^-1\) , and mean metallicity, [Fe/H] = -1.4. From these properties we conclude that Antlia 2 inhabits one of the least dense dark matter (DM) haloes probed to date. Dynamical modelling and tidal-disruption simulations suggest that a combination of a cored DM profile and strong tidal stripping may explain the observed properties of this satellite. The origin of this core may be consistent with aggressive feedback, or may even require alternatives to cold dark matter (such as ultra- light bosons).
@article{2019MNRAS.488.2743T, author = {{Torrealba}, G. and {Belokurov}, V. and {Koposov}, S.~E. and {Li}, T.~S. and {Walker}, M.~G. and {Sanders}, J.~L. and {Geringer-Sameth}, A. and {Zucker}, D.~B. and {Kuehn}, K. and {Evans}, N.~W. and {Dehnen}, W.}, title = {{The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2}}, journal = {\mnras}, keywords = {Galaxy: halo, galaxies: dwarf, galaxies: individual: Antlia 2 Dwarf, Astrophysics - Astrophysics of Galaxies}, year = {2019}, month = sep, volume = {488}, number = {2}, pages = {2743-2766}, doi = {10.1093/mnras/stz1624}, archiveprefix = {arXiv}, eprint = {1811.04082}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.488.2743T}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Transverse kinematics of the Galactic bar-bulge from VVV and Gaia

Jason L. Sanders, Leigh Smith, N. Wyn Evans, and 1 more author

MNRAS, Aug 2019

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We analyse the kinematics of the Galactic bar-bulge using proper motions from the ESO public survey Vista Variables in the Via Lactea (VVV) and the second Gaia data release. Gaia has provided some of the first absolute proper motions within the bulge and the near-infrared VVV multi-epoch catalogue complements Gaia in highly extincted low-latitude regions. We discuss the relative- to-absolute calibration of the VVV proper motions using Gaia. Along lines of sight spanning -10< \textbackslashell / deg< 10 and -10< b/ deg< 5, we probabilistically model the density and velocity distributions as a function of distance of \(∼\) 45 million stars. The transverse velocities confirm the rotation signature of the bar seen in spectroscopic surveys. The differential rotation between the double peaks of the magnitude distribution confirms the X-shaped nature of the bar-bulge. Both transverse velocity components increase smoothly along the near side of the bar towards the Galactic Centre, peak at the Galactic Centre, and decline on the far side. The anisotropy is \(σ\) \(_\(\ell\) \) /\en suremathσ\(_b\) \(≈\) 1.1-1.3 within the bulk of the bar, reducing to 0.9-1.1 when rotational broadening is accounted for, and exhibits a clear X-shaped signature. The vertex deviation in \(\ell\) and b is significant |\(ρ\) \(_\(\ell\) b\) | \(≲\) 0.2, greater on the near side of the bar and produces a quadrupole signature across the bulge indicating approximate radial alignment. We have re-constructed the 3D kinematics from the assumption of triaxiality, finding good agreement with spectroscopic survey results. In the co-rotating frame, we find evidence of bar-supporting x1 orbits and tangential bias in the in-plane dispersion field.
@article{2019MNRAS.487.5188S, author = {{Sanders}, Jason L. and {Smith}, Leigh and {Evans}, N. Wyn and {Lucas}, Philip}, title = {{Transverse kinematics of the Galactic bar-bulge from VVV and Gaia}}, journal = {\mnras}, keywords = {Galaxy: bulge, Galaxy: kinematics and dynamics, Galaxy: structure, Galaxy: centre, Astrophysics - Astrophysics of Galaxies}, year = {2019}, month = aug, volume = {487}, number = {4}, pages = {5188-5208}, doi = {10.1093/mnras/stz1630}, archiveprefix = {arXiv}, eprint = {1903.02008}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.487.5188S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Lessons from the curious case of the ‘fastest’ star in Gaia DR2

D. Boubert, J. Strader, D. Aguado, and 6 more authors

MNRAS, Jun 2019

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Gaia DR2 5932173855446728064 was recently proposed to be unbound from the Milky Way based on the -614.3\(\pm\) 2.5 km s\^{-1} median radial velocity given in Gaia DR2. We obtained eight epochs of spectroscopic follow-up and find a very different median radial velocity of -56.5 \(\pm\) 5.3 km s\^{-1}. If this difference were to be explained by binarity, then the unseen companion would be an intermediate- mass black hole; we therefore argue that the Gaia DR2 radial velocity must be in error. We find it likely that the spectra obtained by Gaia were dominated by the light from a star 4.3 arcsec away, and that, due to the slitless, time delay integration nature of Gaia spectroscopy, this angular offset corresponded to a spurious 620 km s\^{-1} shift in the calcium triplet of the second star. We argue that such unanticipated alignments between stars may account for 105 of the 202 stars with radial velocities faster than 500 km s\^{-1} in Gaia DR2 and propose a quality cut to exclude stars that are susceptible. We propose further cuts to remove stars where the colour photometry is suspect and stars where the radial velocity measurement is based on fewer than four transits, and thus produce an unprecedentedly clean selection of Gaia radial velocities for use in studies of Galactic dynamics.
@article{2019MNRAS.486.2618B, author = {{Boubert}, D. and {Strader}, J. and {Aguado}, D. and {Seabroke}, G. and {Koposov}, S.~E. and {Sanders}, J.~L. and {Swihart}, S. and {Chomiuk}, L. and {Evans}, N.~W.}, title = {{Lessons from the curious case of the `fastest' star in Gaia DR2}}, journal = {\mnras}, keywords = {binaries: general, stars: kinematics and dynamics, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2019}, month = jun, volume = {486}, number = {2}, pages = {2618-2630}, doi = {10.1093/mnras/stz253}, archiveprefix = {arXiv}, eprint = {1901.10460}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.486.2618B}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
MADE: a spectroscopic mass, age, and distance estimator for red giant stars with Bayesian machine learning

Payel Das, and Jason L. Sanders

MNRAS, Mar 2019

Abs ADS arXiv pdf Bib DOI

We present a new approach (MADE) that generates mass, age, and distance estimates of red giant stars from a combination of astrometric, photometric, and spectroscopic data. The core of the approach is a Bayesian artificial neural network (ANN) that learns from and completely replaces stellar isochrones. The ANN is trained using a sample of red giant stars with mass estimates from asteroseismology. A Bayesian isochrone pipeline uses the astrometric, photometric, spectroscopic, and asteroseismology data to determine posterior distributions for the training outputs: mass, age, and distance. Given new inputs, posterior predictive distributions for the outputs are computed, taking into account both input uncertainties, and uncertainties in the ANN parameters. We apply MADE to {\(∼\) }10 000 red giants in the overlap between the 14th data release from the APO Galactic Evolution Experiment (APOGEE) and the Tycho-Gaia astrometric solution (TGAS). The ANN is able to reduce the uncertainty on mass, age, and distance estimates for training- set stars with high output uncertainties allocated through the Bayesian isochrone pipeline. The fractional uncertainties on mass are < 10 per cent and on age are between 10 to 25 per cent. Moreover, the time taken for our ANN to predict masses, ages, and distances for the entire catalogue of APOGEE-TGAS stars is of a similar order of the time taken by the Bayesian isochrone pipeline to run on a handful of stars. Our resulting catalogue clearly demonstrates the expected thick- and thin-disc components in the [M/H]-[\(α\) /M] plane, when examined by age.
@article{2019MNRAS.484..294D, author = {{Das}, Payel and {Sanders}, Jason L.}, title = {{MADE: a spectroscopic mass, age, and distance estimator for red giant stars with Bayesian machine learning}}, journal = {\mnras}, keywords = {methods: data analysis, surveys, Galaxy: evolution, Galaxy: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2019}, month = mar, volume = {484}, number = {1}, pages = {294-304}, doi = {10.1093/mnras/sty2776}, archiveprefix = {arXiv}, eprint = {1804.09596}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2019MNRAS.484..294D}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2018

MNRAS
Isochrone ages for \(∼\) 3 million stars with the second Gaia data release

Jason L. Sanders, and Payel Das

MNRAS, Dec 2018

Abs ADS arXiv pdf Bib DOI

We present a catalogue of distances, masses, and ages for \(∼\) 3 million stars in the second Gaia data release with spectroscopic parameters available from the large spectroscopic surveys: APOGEE, Gaia-ESO, GALAH, LAMOST, RAVE, and SEGUE. We use a Bayesian framework to characterize the probability density functions of distance, mass, and age using photometric, spectroscopic, and astrometric information, supplemented with spectroscopic masses where available for giant stars. Furthermore, we provide posterior extinction estimates (A\(_V\) ) to every star using published extinction maps as a prior input. We provide an appendix with extinction coefficients for Gaia photometry derived from stellar models, which account for variation with intrinsic colour and total extinction. Our pipeline provides output estimates of the spectroscopic parameters, which can be used to inform improved spectroscopic analysis. We complement our catalogues with Galactocentric coordinates and actions with associated uncertainties. As a demonstration of the power of our catalogue, we produce velocity dispersion profiles of the disc separated by age and Galactocentric radius (between 3 and 15 kpc from the Galactic centre). This suggests that the velocity dispersion profiles flatten with radius in the outer Galaxy (>8 kpc) and that at all radii the velocity dispersion follows the smooth power law with age observed in the solar neighbourhood.
@article{2018MNRAS.481.4093S, author = {{Sanders}, Jason L. and {Das}, Payel}, title = {{Isochrone ages for {\(\sim\)\ }3 million stars with the second Gaia data release}}, journal = {\mnras}, keywords = {stars: fundamental parameters, Galaxy: evolution, Galaxy: kinematics and dynamics, Galaxy: stellar content, Galaxy: structure, Astrophysics - Astrophysics of Galaxies, Astrophysics - Solar and Stellar Astrophysics}, year = {2018}, month = dec, volume = {481}, number = {3}, pages = {4093-4110}, doi = {10.1093/mnras/sty2490}, archiveprefix = {arXiv}, eprint = {1806.02324}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2018MNRAS.481.4093S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Discovery of new retrograde substructures: the shards of \(ω\) Centauri?

G. C. Myeong, N. W. Evans, V. Belokurov, and 2 more authors

MNRAS, Aug 2018

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We use the Sloan Digital Sky Survey (SDSS)-Gaia catalogue to search for substructure in the stellar halo. The sample comprises 62 133 halo stars with full phase space coordinates and extends out to heliocentric distances of \(∼\) 10 kpc. As actions are conserved under slow changes of the potential, they permit identification of groups of stars with a common accretion history. We devise a method to identify halo substructures based on their clustering in action space, using metallicity as a secondary check. This is validated against smooth models and numerical constructed stellar haloes from the Aquarius simulations. We identify 21 substructures in the SDSS-Gaia catalogue, including seven high-significance, high-energy and retrograde ones. We investigate whether the retrograde substructures may be material stripped off the atypical globular cluster \(ω\) Centauri. Using a simple model of the accretion of the progenitor of the \(ω\) Centauri, we tentatively argue for the possible association of up to five of our new substructures (labelled Rg1, Rg3, Rg4, Rg6 and Rg7) with this event. This sets a minimum mass of 5 \texttimes 10\(^8\) M\(_\(⊙\) \) for the progenitor, so as to bring \(ω\) Centauri to its current location in action-energy space. Our proposal can be tested by high-resolution spectroscopy of the candidates to look for the unusual abundance patterns possessed by \(ω\) Centauri stars.
@article{2018MNRAS.478.5449M, author = {{Myeong}, G.~C. and {Evans}, N.~W. and {Belokurov}, V. and {Sanders}, J.~L. and {Koposov}, S.~E.}, title = {{Discovery of new retrograde substructures: the shards of {\(\omega\)\ } Centauri?}}, journal = {\mnras}, keywords = {galaxies: kinematics and dynamics, galaxies: structure}, year = {2018}, month = aug, volume = {478}, number = {4}, pages = {5449-5459}, doi = {10.1093/mnras/sty1403}, adsurl = {https://ui.adsabs.harvard.edu/abs/2018MNRAS.478.5449M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Tidal disruption of dwarf spheroidal galaxies: the strange case of Crater II

Jason L. Sanders, N. W. Evans, and W. Dehnen

MNRAS, Aug 2018

Abs ADS arXiv pdf Bib DOI

Dwarf spheroidal galaxies of the Local Group obey a relationship between the line-of-sight velocity dispersion and half-light radius, although there are a number of dwarfs that lie beneath this relation with suppressed velocity dispersion. The most discrepant of these (in the Milky Way) is the ‘feeble giant’ Crater II. Using analytic arguments supported by controlled numerical simulations of tidally stripped flattened two- component dwarf galaxies, we investigate interpretations of Crater II within standard galaxy formation theory. Heavy tidal disruption is necessary to explain the velocity dispersion suppression which is plausible if the proper motion of Crater II is (\(μ\) \(_\(α\) *\) , \(μ\) \(_\(δ\) \) ) = (-0.21 \(\pm\) 0.09, -0.24 \(\pm\) 0.09) mas yr\(^-1\) . Furthermore, we demonstrate that the velocity dispersion of tidally disrupted systems is solely a function of the total mass-loss even for weakly embedded and flattened systems. The half-light radius evolution depends more sensitively on orbital phase and the properties of the dark matter profile. The half-light radius of weakly embedded cusped systems rapidly decreases producing some tension with the Crater II observations. This tension is alleviated by cored dark matter profiles, in which the half-light radius can grow after tidal disruption. The evolution of flattened galaxies is characterized by two competing effects: tidal shocking makes the central regions rounder whilst tidal distortion produces a prolate tidally locked outer envelope. After \(∼\) 70 per cent of the central mass is lost, tidal distortion becomes the dominant effect and the shape of the central regions of the galaxy tends to a universal prolate shape irrespective of the initial shape.
@article{2018MNRAS.478.3879S, author = {{Sanders}, Jason L. and {Evans}, N.~W. and {Dehnen}, W.}, title = {{Tidal disruption of dwarf spheroidal galaxies: the strange case of Crater II}}, journal = {\mnras}, keywords = {galaxies: dwarf, galaxies: evolution, galaxies: fundamental parameters, galaxies: kinematics and dynamics, Local Group, galaxies: structure, Astrophysics - Astrophysics of Galaxies}, year = {2018}, month = aug, volume = {478}, number = {3}, pages = {3879-3889}, doi = {10.1093/mnras/sty1278}, archiveprefix = {arXiv}, eprint = {1802.09537}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2018MNRAS.478.3879S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
A two-parameter family of double-power-law biorthonormal potential-density expansions

Edward J. Lilley, Jason L. Sanders, and N. Wyn Evans

MNRAS, Jul 2018

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We present a two-parameter family of biorthonormal double-power-law potential-density expansions. Both the potential and density are given in a closed analytic form and may be rapidly computed via recurrence relations. We show that this family encompasses all the known analytic biorthonormal expansions: the Zhao expansions (themselves generalizations of ones found earlier by Hernquist & Ostriker and by Clutton-Brock) and the recently discovered Lilley et al. expansion. Our new two-parameter family includes expansions based around many familiar spherical density profiles as zeroth-order models, including the \(γ\) models and the Jaffe model. It also contains a basis expansion that reproduces the famous Navarro-Frenk-White (NFW) profile at zeroth order. The new basis expansions have been found via a systematic methodology which has wide applications in finding other new expansions. In the process, we also uncovered a novel integral transform solution to Poisson’s equation.
@article{2018MNRAS.478.1281L, author = {{Lilley}, Edward J. and {Sanders}, Jason L. and {Evans}, N. Wyn}, title = {{A two-parameter family of double-power-law biorthonormal potential-density expansions}}, journal = {\mnras}, keywords = {galaxies: haloes, galaxies: structure, methods: numerical, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2018}, month = jul, volume = {478}, number = {1}, pages = {1281-1291}, doi = {10.1093/mnras/sty1038}, archiveprefix = {arXiv}, eprint = {1804.11190}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2018MNRAS.478.1281L}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Galaxy halo expansions: a new biorthogonal family of potential-density pairs

Edward J. Lilley, Jason L. Sanders, N. Wyn Evans, and 1 more author

MNRAS, May 2018

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Efficient expansions of the gravitational field of (dark) haloes have two main uses in the modelling of galaxies: first, they provide a compact representation of numerically constructed (or real) cosmological haloes, incorporating the effects of triaxiality, lopsidedness or other distortion. Secondly, they provide the basis functions for self-consistent field expansion algorithms used in the evolution of N-body systems. We present a new family of biorthogonal potential-density pairs constructed using the Hankel transform of the Laguerre polynomials. The lowest order density basis functions are double-power-law profiles cusped like \(ρ\) \(∼\) r\(^-2+1/\(α\) \) at small radii with asymptotic density fall-off like \(ρ\) \(∼\) r\(^-3-1/(2\(α\) )\) . Here, \(α\) is a parameter satisfying \(α\) \(≥\) 1/2. The family therefore spans the range of inner density cusps found in numerical simulations, but has much shallower - and hence more realistic - outer slopes than the corresponding members of the only previously known family deduced by Zhao and exemplified by Hernquist & Ostriker. When \(α\) = 1, the lowest order density profile has an inner density cusp of \(ρ\) \(∼\) r\(^-1\) and an outer density slope of \(ρ\) \(∼\) r\(^-3.5\) , similar to the famous Navarro, Frenk & White (NFW) model. For this reason, we demonstrate that our new expansion provides a more accurate representation of flattened NFW haloes than the competing Hernquist-Ostriker expansion. We utilize our new expansion by analysing a suite of numerically constructed haloes and providing the distributions of the expansion coefficients.
@article{2018MNRAS.476.2092L, author = {{Lilley}, Edward J. and {Sanders}, Jason L. and {Evans}, N. Wyn and {Erkal}, Denis}, title = {{Galaxy halo expansions: a new biorthogonal family of potential-density pairs}}, journal = {\mnras}, keywords = {methods: numerical, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2018}, month = may, volume = {476}, number = {2}, pages = {2092-2109}, doi = {10.1093/mnras/sty296}, archiveprefix = {arXiv}, eprint = {1802.03350}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2018MNRAS.476.2092L}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The super-NFW model: an analytic dynamical model for cold dark matter haloes and elliptical galaxies

Edward J. Lilley, N. Wyn Evans, and Jason L. Sanders

MNRAS, May 2018

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An analytic galaxy model with \(ρ\) \(∼\) r\(^-1\) at small radii and \(ρ\) \(∼\) r\(^-3.5\) at large radii is presented. The asymptotic density fall-off is slower than the Hernquist model, but faster than the Navarro-Frenk-White (NFW) profile for dark matter haloes, and so in accord with recent evidence from cosmological simulations. The model provides the zeroth-order term in a biorthornomal basis function expansion, meaning that axisymmetric, triaxial, and lopsided distortions can easily be added (much like the Hernquist model itself which is the zeroth- order term of the Hernquist-Ostriker expansion). The properties of the spherical model, including analytic distribution functions which are either isotropic, radially anisotropic, or tangentially anisotropic, are discussed in some detail. The analogue of the mass-concentration relation for cosmological haloes is provided.
@article{2018MNRAS.476.2086L, author = {{Lilley}, Edward J. and {Evans}, N. Wyn and {Sanders}, Jason L.}, title = {{The super-NFW model: an analytic dynamical model for cold dark matter haloes and elliptical galaxies}}, journal = {\mnras}, keywords = {galaxies: haloes, galaxies: kinematics and dynamics, dark matter, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2018}, month = may, volume = {476}, number = {2}, pages = {2086-2091}, doi = {10.1093/mnras/sty295}, archiveprefix = {arXiv}, eprint = {1802.03349}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2018MNRAS.476.2086L}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJL
The Milky Way Halo in Action Space

G. C. Myeong, N. W. Evans, V. Belokurov, and 2 more authors

ApJL, Apr 2018

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We analyze the structure of the local stellar halo of the Milky Way using \(∼\) 60000 stars with full phase space coordinates extracted from the SDSS-Gaia catalog. We display stars in action space as a function of metallicity in a realistic axisymmetric potential for the Milky Way Galaxy. The metal-rich population is more distended toward high radial action J \(_ R \) as compared to azimuthal or vertical action, J \(_ \(φ\) \) or J \(_ z \) . It has a mild prograde rotation (< {v}\(_\(\varphi\) \) > \(≈\) 25 {km} {{{s}}}\(^-1\) ), is radially anisotropic and highly flattened, with axis ratio q \(≈\) 0.6-0.7. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation (< {v}\(_\(\varphi\) \) > \(≈\) 50 {km} {{{s}}}\(^-1\) ), a mild radial anisotropy, and a roundish morphology (q \(≈\) 0.9). We identify two further components of the halo in action space. There is a high-energy, retrograde component that is only present in the metal-rich stars. This is suggestive of an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, \(ω\) Centauri. Also visible as a distinct entity in action space is a resonant component, which is flattened and prograde. It extends over a range of metallicities down to [Fe/H] \(≈\) -3. It has a net outward radial velocity < {v}\(_R\) > \(≈\) 12 {km} {{{s}}}\(^-1\) within the solar circle at | z| < 3.5 {kpc}. The existence of resonant stars at such extremely low metallicities has not been seen before.
@article{2018ApJ...856L..26M, author = {{Myeong}, G.~C. and {Evans}, N.~W. and {Belokurov}, V. and {Sanders}, J.~L. and {Koposov}, S.~E.}, title = {{The Milky Way Halo in Action Space}}, journal = {\apjl}, keywords = {Galaxy: halo, Galaxy: stellar content, Galaxy: structure, Astrophysics - Astrophysics of Galaxies}, year = {2018}, month = apr, volume = {856}, number = {2}, eid = {L26}, pages = {L26}, doi = {10.3847/2041-8213/aab613}, archiveprefix = {arXiv}, eprint = {1802.03351}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2018ApJ...856L..26M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2017

MNRAS
The shapes and alignments of the satellites of the Milky Way and Andromeda

Jason L. Sanders, and N. Wyn Evans

MNRAS, Dec 2017

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We measure the intrinsic shapes and alignments of the dwarf spheroidal (dSph) galaxies of the Local Group. We find the dSphs of the Milky Way are intrinsically flatter (mean intrinsic ellipticity \(μ\) \(_E\) \(∼\) 0.6) than those of M31 (\(μ\) \(_E\) \(∼\) 0.5) and that the classical Milky Way dSphs (M\(_V\) < -8.5 mag) are rounder (\(μ\) \(_E\) \(∼\) 0.5) than the ultrafaints (\(μ\) \(_E\) \(∼\) 0.65) whilst in Andromeda (M31) the shapes of the classical and ultrafaint dSphs are very similar. The M31 dSphs are preferentially radially aligned with a dispersion of {\(∼\) }45\textdegree. This signal is driven by the ultrafaint population whilst the classical M31 dSphs are consistent with a random orientation. We compare our results to the Aquarius mock stellar catalogues of Lowing et al. and find the subhalo radial alignment distribution matches the Local Group dSphs results, whilst the Aquarius intrinsic ellipticities are significantly smaller than the data (\(∆\) \ ensuremath⟨E\(⟩\) \(≈\) 0.4). We provide evidence that the major axes of the Milky Way satellites lie within a preferential plane with normal vector pointing towards (\(\ell\) ,b)=(127,5)\textdegree. We associate this preferred direction with the Vast Polar Orbital structure although their respective great circles are offset by {\(∼\) }30\textdegree. No signal in the alignments of the major axes is found in M31, suggesting that the great plane of satellites is formed from recent accretion or chance alignment. Finally, we provide predictions for the discrepancy between the velocity dispersion versus scale radius distributions for the Milky Way and M31 populations and demonstrate that the projection effect from viewing similar populations from two different locations does not account for the discrepancy which is probably caused by increased tidal disruption in M31.
@article{2017MNRAS.472.2670S, author = {{Sanders}, Jason L. and {Evans}, N. Wyn}, title = {{The shapes and alignments of the satellites of the Milky Way and Andromeda}}, journal = {\mnras}, keywords = {galaxies: dwarf, galaxies: fundamental parameters, galaxies: kinematics and dynamics, Local Group, galaxies: structure, Astrophysics - Astrophysics of Galaxies}, year = {2017}, month = dec, volume = {472}, number = {3}, pages = {2670-2685}, doi = {10.1093/mnras/stx2116}, archiveprefix = {arXiv}, eprint = {1708.06526}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2017MNRAS.472.2670S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Climbing the cosmic ladder with stellar twins in RAVE with Gaia

P. Jofré, G. Traven, K. Hawkins, and 17 more authors

MNRAS, Dec 2017

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We apply the twin method to determine parallaxes to 232 545 stars of the RAVE survey using the parallaxes of Gaia DR1 as a reference. To search for twins in this large data set, we apply the t-student stochastic neighbour embedding projection that distributes the data according to their spectral morphology on a two-dimensional map. From this map, we choose the twin candidates for which we calculate a \(χ\) \(^2\) to select the best sets of twins. Our results show a competitive performance when compared to other model-dependent methods relying on stellar parameters and isochrones. The power of the method is shown by finding that the accuracy of our results is not significantly affected if the stars are normal or peculiar since the method is model free. We find twins for 60 per cent of the RAVE sample that are not contained in Tycho-Gaia Astrometric Solution (TGAS) or that have TGAS uncertainties that are larger than 20 per cent. We could determine parallaxes with typical errors of 28 per cent. We provide a complementary data set for the RAVE stars not covered by TGAS, or that have TGAS uncertainties which are larger than 20 per cent, with model-free parallaxes scaled to the Gaia measurements.
@article{2017MNRAS.472.2517J, author = {{Jofr{\'e}}, P. and {Traven}, G. and {Hawkins}, K. and {Gilmore}, G. and {Sanders}, J.~L. and {M{\"a}dler}, T. and {Steinmetz}, M. and {Kunder}, A. and {Kordopatis}, G. and {McMillan}, P. and {Bienaym{\'e}}, O. and {Bland-Hawthorn}, J. and {Gibson}, B.~K. and {Grebel}, E.~K. and {Munari}, U. and {Navarro}, J. and {Parker}, Q. and {Reid}, W. and {Seabroke}, G. and {Zwitter}, T.}, title = {{Climbing the cosmic ladder with stellar twins in RAVE with Gaia}}, journal = {\mnras}, keywords = {methods: statistical, techniques: spectroscopic, stars: distances, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2017}, month = dec, volume = {472}, number = {3}, pages = {2517-2533}, doi = {10.1093/mnras/stx1877}, archiveprefix = {arXiv}, eprint = {1705.11049}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2017MNRAS.472.2517J}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The Gaia-ESO Survey: dynamical models of flattened, rotating globular clusters

S. M. R. Jeffreson, J. L. Sanders, N. W. Evans, and 21 more authors

MNRAS, Aug 2017

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We present a family of self-consistent axisymmetric rotating globular cluster models which are fitted to spectroscopic data for NGC 362, NGC 1851, NGC 2808, NGC 4372, NGC 5927 and NGC 6752 to provide constraints on their physical and kinematic properties, including their rotation signals. They are constructed by flattening Modified Plummer profiles, which have the same asymptotic behaviour as classical Plummer models, but can provide better fits to young clusters due to a slower turnover in the density profile. The models are in dynamical equilibrium as they depend solely on the action variables. We employ a fully Bayesian scheme to investigate the uncertainty in our model parameters (including mass-to-light ratios and inclination angles) and evaluate the Bayesian evidence ratio for rotating to non-rotating models. We find convincing levels of rotation only in NGC 2808. In the other clusters, there is just a hint of rotation (in particular, NGC 4372 and NGC 5927), as the data quality does not allow us to draw strong conclusions. Where rotation is present, we find that it is confined to the central regions, within radii of R \(≤\) 2r\(_h\) . As part of this work, we have developed a novel q-Gaussian basis expansion of the line-of-sight velocity distributions, from which general models can be constructed via interpolation on the basis coefficients.
@article{2017MNRAS.469.4740J, author = {{Jeffreson}, S.~M.~R. and {Sanders}, J.~L. and {Evans}, N.~W. and {Williams}, A.~A. and {Gilmore}, G.~F. and {Bayo}, A. and {Bragaglia}, A. and {Casey}, A.~R. and {Flaccomio}, E. and {Franciosini}, E. and {Hourihane}, A. and {Jackson}, R.~J. and {Jeffries}, R.~D. and {Jofr{\'e}}, P. and {Koposov}, S. and {Lardo}, C. and {Lewis}, J. and {Magrini}, L. and {Morbidelli}, L. and {Pancino}, E. and {Randich}, S. and {Sacco}, G.~G. and {Worley}, C.~C. and {Zaggia}, S.}, title = {{The Gaia-ESO Survey: dynamical models of flattened, rotating globular clusters}}, journal = {\mnras}, keywords = {methods: numerical, stars: kinematics and dynamics, globular clusters: general, galaxies: star clusters: general, Astrophysics - Astrophysics of Galaxies}, year = {2017}, month = aug, volume = {469}, number = {4}, pages = {4740-4762}, doi = {10.1093/mnras/stx1152}, archiveprefix = {arXiv}, eprint = {1704.07833}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2017MNRAS.469.4740J}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
A halo substructure in Gaia Data Release 1

G. C. Myeong, N. W. Evans, V. Belokurov, and 2 more authors

MNRAS, Jul 2017

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We identify a halo substructure in the Tycho Gaia Astrometric Solution (TGAS) data set, cross-matched with the Radial Velocity Experiment (RAVE-on) data release. After quality cuts, the stars with large radial action (J\(_R\) > 800 km s\(^-1\) kpc) are extracted. A subset of these stars is clustered in longitude and velocity and can be selected with further cuts. The 14 stars are centred on (X, Y, Z) \(≈\) (9.0, -1.0, -0.6) kpc and form a coherently moving structure in the halo with median (v\(_R\) , v\(_\(φ\) \) , v\(_z\) ) = (167.33, 0.86, -94.85) km s\(^-1\) . They are all metal-poor giants with median [Fe/H] = -0.83. To guard against the effects of distance errors, we compute spectrophotometric distances for 8 out of the 14 stars where this is possible. We find that six of the stars are still comoving. These six stars also have a much tighter [Fe/H] distribution \(∼\) -0.7 with one exception ([Fe/H] = -2.12). We conclude that the existence of the comoving cluster is stable against changes in distance estimation and conjecture that this is the dissolving remnant of a globular cluster accreted long ago.
@article{2017MNRAS.469L..78M, author = {{Myeong}, G.~C. and {Evans}, N.~W. and {Belokurov}, V. and {Koposov}, S.~E. and {Sanders}, J.~L.}, title = {{A halo substructure in Gaia Data Release 1}}, journal = {\mnras}, keywords = {galaxies: kinematics and dynamics, galaxies: structure, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2017}, month = jul, volume = {469}, number = {1}, pages = {L78-L82}, doi = {10.1093/mnrasl/slx051}, archiveprefix = {arXiv}, eprint = {1704.01363}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2017MNRAS.469L..78M}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Reflection symmetries of Isolated Self-consistent Stellar Systems

J. An, N. W. Evans, and J. L. Sanders

MNRAS, May 2017

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Isolated, steady-state galaxies correspond to equilibrium solutions of the Poisson-Vlasov system. We show that (I) all galaxies with a distribution function (DF) depending on energy alone f(E) must be spherically symmetric, and (II) all axisymmetric galaxies with a DF depending on energy and the angular momentum component parallel to the symmetry axis f(E, L\(_z\) ) must also be reflection-symmetric about the plane z = 0. The former result is known, whilst the latter result is new. These results are subsumed into the Symmetry Theorem, which specifies how the symmetries of the DF in configuration or velocity space can control the planes of reflection symmetries of the ensuing stellar system.
@article{2017MNRAS.467.1281A, author = {{An}, J. and {Evans}, N.~W. and {Sanders}, J.~L.}, title = {{Reflection symmetries of Isolated Self-consistent Stellar Systems}}, journal = {\mnras}, keywords = {galaxies: kinematics and dynamics, galaxies: structure, Astrophysics - Astrophysics of Galaxies}, year = {2017}, month = may, volume = {467}, number = {2}, pages = {1281-1286}, doi = {10.1093/mnras/stx195}, archiveprefix = {arXiv}, eprint = {1610.01701}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2017MNRAS.467.1281A}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJ
The RAVE-on Catalog of Stellar Atmospheric Parameters and Chemical Abundances for Chemo-dynamic Studies in the Gaia Era

Andrew R. Casey, Keith Hawkins, David W. Hogg, and 24 more authors

ApJ, May 2017

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The orbits, atmospheric parameters, chemical abundances, and ages of individual stars in the Milky Way provide the most comprehensive illustration of galaxy formation available. The Tycho-Gaia Astrometric Solution (TGAS) will deliver astrometric parameters for the largest ever sample of Milky Way stars, though its full potential cannot be realized without the addition of complementary spectroscopy. Among existing spectroscopic surveys, the RAdial Velocity Experiment (RAVE) has the largest overlap with TGAS (\(≳\) 200,000 stars). We present a data-driven re-analysis of 520,781 RAVE spectra using The Cannon. For red giants, we build our model using high- fidelity APOGEE stellar parameters and abundances for stars that overlap with RAVE. For main sequence and sub-giant stars, our model uses stellar parameters from the K2/EPIC. We derive and validate effective temperature T \(_eff\) , surface gravity log g, and chemical abundances of up to seven elements (O, Mg, Al, Si, Ca, Fe, and Ni). We report a total of 1,685,851 elemental abundances with a typical precision of 0.07 dex, a substantial improvement over previous RAVE data releases. The synthesis of RAVE-on and TGAS is the most powerful data set for chemo-dynamic analyses of the Milky Way ever produced.
@article{2017ApJ...840...59C, author = {{Casey}, Andrew R. and {Hawkins}, Keith and {Hogg}, David W. and {Ness}, Melissa and {Rix}, Hans-Walter and {Kordopatis}, Georges and {Kunder}, Andrea and {Steinmetz}, Matthias and {Koposov}, Sergey and {Enke}, Harry and {Sanders}, Jason and {Gilmore}, Gerry and {Zwitter}, Toma{\v{z}} and {Freeman}, Kenneth C. and {Casagrande}, Luca and {Matijevi{\v{c}}}, Gal and {Seabroke}, George and {Bienaym{\'e}}, Olivier and {Bland-Hawthorn}, Joss and {Gibson}, Brad K. and {Grebel}, Eva K. and {Helmi}, Amina and {Munari}, Ulisse and {Navarro}, Julio F. and {Reid}, Warren and {Siebert}, Arnaud and {Wyse}, Rosemary}, title = {{The RAVE-on Catalog of Stellar Atmospheric Parameters and Chemical Abundances for Chemo-dynamic Studies in the Gaia Era}}, journal = {\apj}, keywords = {stars: abundances, stars: fundamental parameters, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies}, year = {2017}, month = may, volume = {840}, number = {1}, eid = {59}, pages = {59}, doi = {10.3847/1538-4357/aa69c2}, archiveprefix = {arXiv}, eprint = {1609.02914}, primaryclass = {astro-ph.SR}, adsurl = {https://ui.adsabs.harvard.edu/abs/2017ApJ...840...59C}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Linear perturbation theory for tidal streams and the small-scale CDM power spectrum

Jo Bovy, Denis Erkal, and Jason L. Sanders

MNRAS, Apr 2017

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Tidal streams in the Milky Way are sensitive probes of the population of low-mass dark matter subhaloes predicted in cold dark matter (CDM) simulations. We present a new calculus for computing the effect of subhalo fly-bys on cold streams based on the action- angle representation of streams. The heart of this calculus is a line-of-parallel-angle approach that calculates the perturbed distribution function of a stream segment by undoing the effect of all relevant impacts. This approach allows one to compute the perturbed stream density and track in any coordinate system in minutes for realizations of the subhalo distribution down to 10\(^5\) M\(_\(⊙\) \) , accounting for the stream’s internal dispersion and overlapping impacts. We study the statistical properties of density and track fluctuations with large suites of simulations of the effect of subhalo fly- bys. The one-dimensional density and track power spectra along the stream trace the subhalo mass function, with higher mass subhaloes producing power only on large scales, while lower mass subhaloes cause structure on smaller scales. We also find significant density and track bispectra that are observationally accessible. We further demonstrate that different projections of the track all reflect the same pattern of perturbations, facilitating their observational measurement. We apply this formalism to data for the Pal 5 stream and make a first rigorous determination of 10\^{+11}_{-6} dark matter subhaloes with masses between 10\(^6.5\) and 10\(^9\) M\(_\(⊙\) \) within 20 kpc from the Galactic centre [corresponding to 1.4\^{+1.6}_{-0.9} times the number predicted by CDM-only simulations or to f\(_sub\) (r < 20 kpc) \(≈\) 0.2 per cent] assuming that the Pal 5 stream is 5 Gyr old. Improved data will allow measurements of the subhalo mass function down to 10\(^5\) M\(_\(⊙\) \) , thus definitively testing whether dark matter is clumpy on the smallest scales relevant for galaxy formation.
@article{2017MNRAS.466..628B, author = {{Bovy}, Jo and {Erkal}, Denis and {Sanders}, Jason L.}, title = {{Linear perturbation theory for tidal streams and the small-scale CDM power spectrum}}, journal = {\mnras}, keywords = {Galaxy: fundamental parameters, Galaxy: halo, Galaxy: kinematics and dynamics, Galaxy: structure, dark matter, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2017}, month = apr, volume = {466}, number = {1}, pages = {628-668}, doi = {10.1093/mnras/stw3067}, archiveprefix = {arXiv}, eprint = {1606.03470}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2017MNRAS.466..628B}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2016

MNRAS
The number and size of subhalo-induced gaps in stellar streams

Denis Erkal, Vasily Belokurov, Jo Bovy, and 1 more author

MNRAS, Nov 2016

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Ample observational capabilities exist today to detect the small density perturbations that low-mass dark matter subhaloes impart on stellar streams from disrupting Galactic satellites. In anticipation of these observations, we investigate the expected number and size of gaps by combining an analytic prescription for gap evolution on circular orbits with the flux of subhaloes near the stream. We explore the distribution of gap sizes and depths for a typical cold stream around the Milky Way and find that for a given stream age and gap depth, each subhalo mass produces a characteristic gap size. For a stream with an age of a few Gyr, orbiting at a distance of 10-20 kpc from the Galactic centre, even modest subhaloes with a mass of 10\(^6\) -10\(^7\) M\(_\(⊙\) \) produce gaps with sizes that are of the order of several degrees. We consider the number and distribution of gap sizes created by subhaloes with masses 10\(^5\) -10\(^9\) M\(_\(⊙\) \) , accounting for the expected depletion of subhaloes by the Milky Way disc, and present predictions for six cold streams around the Milky Way. For Pal 5, we forecast 0.7 gaps with a density depletion of at least 25 per cent and a typical gap size of 8\textdegree. Thus, there appears to be no tension between the recent non-detection of density depletions in the Pal 5 tidal tails and \(Λ\) CDM expectations. These predictions can be used to guide the scale of future gap searches.
@article{2016MNRAS.463..102E, author = {{Erkal}, Denis and {Belokurov}, Vasily and {Bovy}, Jo and {Sanders}, Jason L.}, title = {{The number and size of subhalo-induced gaps in stellar streams}}, journal = {\mnras}, keywords = {Galaxy: fundamental parameters, galaxies: haloes, galaxies: structure, dark matter, Astrophysics - Astrophysics of Galaxies}, year = {2016}, month = nov, volume = {463}, number = {1}, pages = {102-119}, doi = {10.1093/mnras/stw1957}, archiveprefix = {arXiv}, eprint = {1606.04946}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016MNRAS.463..102E}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
ApJL
Mass Estimators for Flattened Dispersion-supported Galaxies

Jason L. Sanders, and N. Wyn Evans

ApJL, Oct 2016

Abs ADS arXiv pdf Bib DOI

We investigate the reliability of mass estimators based on the observable velocity dispersion and half-light radius R \(_h\) for dispersion-supported galaxies. We show how to extend them to flattened systems and provide simple formulae for the mass within an ellipsoid under the assumption the dark-matter density and the stellar density are stratified on the same self-similar ellipsoids. We demonstrate explicitly that the spherical mass estimators give accurate values for the mass within the half- light ellipsoid, provided R \(_h\) is replaced by its “circularized” analog {R}\(_{{h\) }}\textbackslashsqrt{1-ɛ }. We provide a mathematical justification for this surprisingly simple and effective workaround. It means, for example, that the mass-to- light ratios are valid not just when the light and dark matter are spherically distributed, but also when they are flattened on ellipsoids of the same constant shape.
@article{2016ApJ...830L..26S, author = {{Sanders}, Jason L. and {Evans}, N. Wyn}, title = {{Mass Estimators for Flattened Dispersion-supported Galaxies}}, journal = {\apjl}, keywords = {dark matter, galaxies: dwarf, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics}, year = {2016}, month = oct, volume = {830}, number = {2}, eid = {L26}, pages = {L26}, doi = {10.3847/2041-8205/830/2/L26}, archiveprefix = {arXiv}, eprint = {1609.05903}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016ApJ...830L..26S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
PRD
Indirect dark matter detection for flattened dwarf galaxies

Jason L. Sanders, N. Wyn Evans, Alex Geringer-Sameth, and 1 more author

PRD, Sep 2016

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Gamma-ray experiments seeking to detect evidence of dark matter annihilation in dwarf spheroidal galaxies require knowledge of the distribution of dark matter within these systems. We analyze the effects of flattening on the annihilation (J) and decay (D) factors of dwarf spheroidal galaxies with both analytic and numerical methods. Flattening has two consequences: first, there is a geometric effect as the squeezing (or stretching) of the dark matter distribution enhances (or diminishes) the J-factor; second, the line of sight velocity dispersion of stars must hold up the flattened baryonic component in the flattened dark matter halo. We provide analytic formulas and a simple numerical approach to estimate the correction to the J- and D-factors required over simple spherical modeling. The formulas are validated with a series of equilibrium models of flattened stellar distributions embedded in flattened dark-matter distributions. We compute corrections to the J- and D-factors for the Milky Way dwarf spheroidal galaxies under the assumption that they are all prolate or all oblate and find that the hierarchy of J-factors for the dwarf spheroidals is slightly altered (typical correction factors for an ellipticity of 0.4 are 0.75 for the oblate case and 1.6 for the prolate case). We demonstrate that spherical estimates of the D-factors are very insensitive to the flattening and introduce uncertainties significantly less than the uncertainties in the D-factors from the other observables for all the dwarf spheroidals (for example, +10 per cent/-3 per cent for a typical ellipticity of 0.4). We conclude by investigating the spread in correction factors produced by triaxial figures and provide uncertainties in the J-factors for the dwarf spheroidals using different physically motivated assumptions for their intrinsic shape and axis alignments. We find that the uncertainty in the J-factors due to triaxiality increases with the observed ellipticity and, in general, introduces uncertainties of a factor of 2 in the J-factors. We discuss our results in light of the reported gamma-ray signal from the highly flattened ultrafaint Reticulum II. Tables of the J- and D-factors for the Milky Way dwarf spheroidal galaxies are provided (assuming an oblate or prolate structure) along with a table of the uncertainty on these factors arising from the unknown triaxiality.
@article{2016PhRvD..94f3521S, author = {{Sanders}, Jason L. and {Evans}, N. Wyn and {Geringer-Sameth}, Alex and {Dehnen}, Walter}, title = {{Indirect dark matter detection for flattened dwarf galaxies}}, journal = {\prd}, keywords = {Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - High Energy Astrophysical Phenomena}, year = {2016}, month = sep, volume = {94}, number = {6}, eid = {063521}, pages = {063521}, doi = {10.1103/PhysRevD.94.063521}, archiveprefix = {arXiv}, eprint = {1604.05493}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016PhRvD..94f3521S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Stray, swing and scatter: angular momentum evolution of orbits and streams in aspherical potentials

Denis Erkal, Jason L. Sanders, and Vasily Belokurov

MNRAS, Sep 2016

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In aspherical potentials orbital planes continuously evolve. The gravitational torques impel the angular momentum vector to precess, that is to slowly stray around the symmetry axis, and nutate, I.e. swing up and down periodically in the perpendicular direction. This familiar orbital pole motion - if detected and measured - can reveal the shape of the underlying gravitational potential, the quantity only crudely gauged in the Galaxy so far. Here we demonstrate that the debris poles of stellar tidal streams show a very similar straying and swinging behaviour, and give analytic expressions to link the amplitude and the frequency of the pole evolution to the flattening of the dark matter distribution. While these results are derived for near- circular orbits, we show they are also valid for eccentric orbits. Most importantly, we explain how the differential orbital plane precession leads to the broadening of the stream and show that streams on polar orbits ought to scatter faster. We provide expressions for the stream width evolution as a function of the axisymmetric potential flattening and the angle from the symmetry plane and prove that our models are in good agreement with streams produced in N-body simulations. Interestingly, the same intuition applies to streams whose progenitors are on short- or long-axis loops in a triaxial potential. Finally, we present a compilation of the Galactic cold stream data, and discuss how the simple picture developed here, along with stream modelling, can be used to constrain the symmetry axes and flattening of the Milky Way.
@article{2016MNRAS.461.1590E, author = {{Erkal}, Denis and {Sanders}, Jason L. and {Belokurov}, Vasily}, title = {{Stray, swing and scatter: angular momentum evolution of orbits and streams in aspherical potentials}}, journal = {\mnras}, keywords = {galaxies: haloes, galaxies: structure, dark matter, Astrophysics - Astrophysics of Galaxies}, year = {2016}, month = sep, volume = {461}, number = {2}, pages = {1590-1604}, doi = {10.1093/mnras/stw1400}, archiveprefix = {arXiv}, eprint = {1603.08922}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016MNRAS.461.1590E}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

Astronomische Nachrichten

Chemodynamical modelling of the Milky Way

J. Binney, and J. L. Sanders

Astronomische Nachrichten, Sep 2016

Abs ADS arXiv pdf Bib DOI

@article{2016AN....337..939B,
  author = {{Binney}, J. and {Sanders}, J.~L.},
  title = {{Chemodynamical modelling of the Milky Way}},
  journal = {Astronomische Nachrichten},
  keywords = {Astrophysics - Astrophysics of Galaxies},
  year = {2016},
  month = sep,
  volume = {337},
  number = {8-9},
  pages = {939},
  doi = {10.1002/asna.201612403},
  archiveprefix = {arXiv},
  eprint = {1511.08480},
  primaryclass = {astro-ph.GA},
  adsurl = {https://ui.adsabs.harvard.edu/abs/2016AN....337..939B},
  adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

MNRAS
Shepherding tidal debris with the Galactic bar: the Ophiuchus stream

Kohei Hattori, Denis Erkal, and Jason L. Sanders

MNRAS, Jul 2016

Abs ADS arXiv pdf Bib DOI

The dynamics of stellar streams in rotating barred potentials is explained for the first time. Naturally, neighbouring stream stars reach pericentre at slightly different times. In the presence of a rotating bar, these neighbouring stream stars experience different bar orientations during pericentric passage and hence each star receives a different torque from the bar. These differing torques reshape the angular momentum and energy distribution of stars in the stream, which in turn changes the growth rate of the stream. For a progenitor orbiting in the same sense as the bar’s rotation and satisfying a resonance condition, the resultant stream can be substantially shorter or longer than expected, depending on whether the pericentric passages of the progenitor occur along the bar’s minor or major axis, respectively. We present a full discussion of this phenomenon focusing mainly on streams confined to the Galactic plane. In stark contrast with the evolution in static potentials, which give rise to streams that grow steadily in time, rotating barred potentials can produce dynamically old, short streams. This challenges the traditional viewpoint that the inner halo necessarily consists of well phase-mixed material whilst the tidally disrupted structures in the outer halo are more spatially coherent. We argue that this mechanism may play an important role in explaining the mysteriously short Ophiuchus stream that was recently discovered near the bulge region of the Milky Way.
@article{2016MNRAS.460..497H, author = {{Hattori}, Kohei and {Erkal}, Denis and {Sanders}, Jason L.}, title = {{Shepherding tidal debris with the Galactic bar: the Ophiuchus stream}}, journal = {\mnras}, keywords = {Galaxy: bulge, Galaxy: evolution, Galaxy: kinematics and dynamics, Galaxy: structure, Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - Solar and Stellar Astrophysics}, year = {2016}, month = jul, volume = {460}, number = {1}, pages = {497-512}, doi = {10.1093/mnras/stw1006}, archiveprefix = {arXiv}, eprint = {1512.04536}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016MNRAS.460..497H}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
PRD
Simple J-factors and D-factors for indirect dark matter detection

N. W. Evans, J. L. Sanders, and Alex Geringer-Sameth

PRD, May 2016

Abs ADS arXiv pdf Bib DOI

J-factors (or D-factors) describe the distribution of dark matter in an astrophysical system and determine the strength of the signal provided by annihilating (or decaying) dark matter respectively. We provide simple analytic formulas to calculate the J-factors for spherical cusps obeying the empirical relationship between enclosed mass, velocity dispersion and half-light radius. We extend the calculation to the spherical Navarro-Frenk-White model, and demonstrate that our new formulas give accurate results in comparison to more elaborate Jeans models driven by Markov chain Monte Carlo methods. Of the known ultrafaint dwarf spheroidals, we show that Ursa Major II, Reticulum II, Tucana II and Horologium I have the largest J-factors and so provide the most promising candidates for indirect dark matter detection experiments. Amongst the classical dwarfs, Draco, Sculptor and Ursa Minor have the highest J-factors. We show that the behavior of the J-factor as a function of integration angle can be inferred for general dark halo models with inner slope \(γ\) and outer slope \(β\) . The central and asymptotic behavior of the J-factor curves are derived as a function of the dark halo properties. Finally, we show that models obeying the empirical relation on enclosed mass and velocity dispersion have J-factors that are most robust at the integration angle equal to the projected half-light radius of the dwarf spheroidal (dSph) divided by heliocentric distance. For most of our results, we give the extension to the D-factor which is appropriate for the decaying dark matter picture.
@article{2016PhRvD..93j3512E, author = {{Evans}, N.~W. and {Sanders}, J.~L. and {Geringer-Sameth}, Alex}, title = {{Simple J-factors and D-factors for indirect dark matter detection}}, journal = {\prd}, keywords = {Astrophysics - Astrophysics of Galaxies, Astrophysics - Cosmology and Nongalactic Astrophysics, Astrophysics - High Energy Astrophysical Phenomena}, year = {2016}, month = may, volume = {93}, number = {10}, eid = {103512}, pages = {103512}, doi = {10.1103/PhysRevD.93.103512}, archiveprefix = {arXiv}, eprint = {1604.05599}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016PhRvD..93j3512E}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Dynamics of stream-subhalo interactions

Jason L. Sanders, Jo Bovy, and Denis Erkal

MNRAS, Apr 2016

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We develop a formalism for modelling the impact of dark matter subhaloes on cold thin streams. Our formalism models the formation of a gap in a stream in angle-frequency space and is able to handle general stream and impact geometry. We analyse an N-body simulation of a cold stream formed from a progenitor on an eccentric orbit in an axisymmetric potential, which is perturbed by a direct impact from a 10\(^8\) M\(_\(⊙\) \) subhalo, and produce a complete generative model of the perturbed stream that matches the simulation well at a range of times. We show how the results in angle-frequency space can be related to physical properties of the gaps and that previous results for more constrained simulations are recovered. We demonstrate how our results are dependent upon the mass of the subhalo and the location of the impact along the stream. We find that gaps formed far downstream grow more rapidly than those closer to the progenitor due to the more ordered nature of the stream members far from the progenitor. Additionally, we show that the minimum gap density plateaus in time at a value that decreases with increasing subhalo mass.
@article{2016MNRAS.457.3817S, author = {{Sanders}, Jason L. and {Bovy}, Jo and {Erkal}, Denis}, title = {{Dynamics of stream-subhalo interactions}}, journal = {\mnras}, keywords = {Galaxy: halo, Galaxy: kinematics and dynamics, Galaxy: structure, cosmology: theory, dark matter, Astrophysics - Astrophysics of Galaxies}, year = {2016}, month = apr, volume = {457}, number = {4}, pages = {3817-3835}, doi = {10.1093/mnras/stw232}, archiveprefix = {arXiv}, eprint = {1510.03426}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016MNRAS.457.3817S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
A review of action estimation methods for galactic dynamics

Jason L. Sanders, and James Binney

MNRAS, Apr 2016

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We review the available methods for estimating actions, angles and frequencies of orbits in both axisymmetric and triaxial potentials. The methods are separated into two classes. Unless an orbit has been trapped by a resonance, convergent, or iterative, methods are able to recover the actions to arbitrarily high accuracy given sufficient computing time. Faster non-convergent methods rely on the potential being sufficiently close to a separable potential, and the accuracy of the action estimate cannot be improved through further computation. We critically compare the accuracy of the methods and the required computation time for a range of orbits in an axisymmetric multicomponent Galactic potential. We introduce a new method for estimating actions that builds on the adiabatic approximation of Schönrich & Binney and discuss the accuracy required for the actions, angles and frequencies using suitable distribution functions for the thin and thick discs, the stellar halo and a star stream. We conclude that for studies of the disc and smooth halo component of the Milky Way, the most suitable compromise between speed and accuracy is the Stäckel Fudge, whilst when studying streams the non- convergent methods do not offer sufficient accuracy and the most suitable method is computing the actions from an orbit integration via a generating function. All the software used in this study can be downloaded from https://github.com/jls713/tact.
@article{2016MNRAS.457.2107S, author = {{Sanders}, Jason L. and {Binney}, James}, title = {{A review of action estimation methods for galactic dynamics}}, journal = {\mnras}, keywords = {methods: numerical, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2016}, month = apr, volume = {457}, number = {2}, pages = {2107-2121}, doi = {10.1093/mnras/stw106}, archiveprefix = {arXiv}, eprint = {1511.08213}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016MNRAS.457.2107S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
The alignment of the second velocity moment tensor in galaxies

N. W. Evans, J. L. Sanders, A. A. Williams, and 3 more authors

MNRAS, Mar 2016

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We show that provided the principal axes of the second velocity moment tensor of a stellar population are generally unequal and are oriented perpendicular to a set of orthogonal surfaces at each point, then those surfaces must be confocal quadric surfaces and the potential must be separable or Stäckel. This is true under the mild assumption that the even part of the distribution function (DF) is invariant under time reversal v\(_I\) \textrightarrow -v\(_I\) of each velocity component. In particular, if the second velocity moment tensor is everywhere exactly aligned in spherical polar coordinates, then the potential must be of separable or Stäckel form (excepting degenerate cases where two or more of the semiaxes of ellipsoid are everywhere the same). The theorem also has restrictive consequences for alignment in cylindrical polar coordinates, which is used in the popular Jeans Anisotropic Models (JAM) of Cappellari. We analyse data on the radial velocities and proper motions of a sample of \(∼\) 7300 stars in the stellar halo of the Milky Way. We provide the distributions of the tilt angles or misalignments from both the spherical polar coordinate systems. We show that in this sample the misalignment is always small (usually within 3\textdegree) for Galactocentric radii between \(∼\) 6 and \(∼\) 11 kpc. The velocity anisotropy is very radially biased (\(β\) \(≈\) 0.7), and almost invariant across the volume in our study. Finally, we construct a triaxial stellar halo in a triaxial NFW dark matter halo using a made-to-measure method. Despite the triaxiality of the potential, the velocity ellipsoid of the stellar halo is nearly spherically aligned within \(∼\) 6\textdegree for large regions of space, particularly outside the scale radius of the stellar halo. We conclude that the second velocity moment ellipsoid can be close to spherically aligned for a much wider class of potentials than the strong constraints that arise from exact alignment might suggest.
@article{2016MNRAS.456.4506E, author = {{Evans}, N.~W. and {Sanders}, J.~L. and {Williams}, A.~A. and {An}, J. and {Lynden-Bell}, D. and {Dehnen}, W.}, title = {{The alignment of the second velocity moment tensor in galaxies}}, journal = {\mnras}, keywords = {Galaxy: halo, galaxies: haloes, galaxies: kinematics and dynamics, dark matter, Astrophysics - Astrophysics of Galaxies}, year = {2016}, month = mar, volume = {456}, number = {4}, pages = {4506-4523}, doi = {10.1093/mnras/stv2729}, archiveprefix = {arXiv}, eprint = {1509.08794}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2016MNRAS.456.4506E}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2015

MNRAS
Self-consistent triaxial models

Jason L. Sanders, and N. Wyn Evans

MNRAS, Nov 2015

Abs ADS arXiv pdf Bib DOI

We present self-consistent triaxial stellar systems that have analytic distribution functions (DFs) expressed in terms of the actions. These provide triaxial density profiles with cores or cusps at the centre. They are the first self-consistent triaxial models with analytic DFs suitable for modelling giant ellipticals and dark haloes. Specifically, we study triaxial models that reproduce the Hernquist profile from Williams & Evans, as well as flattened isochrones of the form proposed by Binney. We explore the kinematics and orbital structure of these models in some detail. The models typically become more radially anisotropic on moving outwards, have velocity ellipsoids aligned in Cartesian coordinates in the centre and aligned in spherical polar coordinates in the outer parts. In projection, the ellipticity of the isophotes and the position angle of the major axis of our models generally changes with radius. So, a natural application is to elliptical galaxies that exhibit isophote twisting. As triaxial Stäckel models do not show isophote twists, our DFs are the first to generate mass density distributions that do exhibit this phenomenon, typically with a gradient of \(≈\) 10\textdegree/effective radius, which is comparable to the data. Triaxiality is a natural consequence of models that are susceptible to the radial orbit instability. We show how a family of spherical models with anisotropy profiles that transition from isotropic at the centre to radially anisotropic becomes unstable when the outer anisotropy is made sufficiently radial. Models with a larger outer anisotropy can be constructed but are found to be triaxial. We argue that the onset of the radial orbit instability can be identified with the transition point when adiabatic relaxation yields strongly triaxial rather than weakly spherical endpoints.
@article{2015MNRAS.454..299S, author = {{Sanders}, Jason L. and {Evans}, N. Wyn}, title = {{Self-consistent triaxial models}}, journal = {\mnras}, keywords = {methods: analytical, methods: numerical, Galaxy: kinematics and dynamics, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2015}, month = nov, volume = {454}, number = {1}, pages = {299-314}, doi = {10.1093/mnras/stv1898}, archiveprefix = {arXiv}, eprint = {1507.04129}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2015MNRAS.454..299S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Extended distribution functions for our Galaxy

Jason L. Sanders, and James Binney

MNRAS, Jun 2015

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We extend models of our Galaxy based on distribution functions that are analytic functions of the action integrals to extended distribution functions (EDFs), which have an analytic dependence on metallicity as well. We use a simple, but physically motivated, functional forms for the metallicity of the interstellar medium as a function of radius and time and for the star formation rate, and a model for the diffusion of stars through phase space to suggest the required functional form of an EDF. We introduce a simple prescription for radial migration that preserves the overall profile of the disc while allowing individual stars to migrate throughout the disc. Our models explicitly consider the thin and thick discs as two distinct components separated in age. We show how an EDF can be used to incorporate realistic selection functions in models, and to construct mock catalogues of observed samples. We show that the selection function of the Geneva-Copenhagen Survey (GCS) biases in favour of young stars, which have atypically small random velocities. With the selection function taken into account our models produce good fits of the GCS data in chemo-dynamical space and the Gilmore & Reid (1983) density data. From our EDF, we predict the structure of the Sloan Extension for Galactic Understanding and Exploration G-dwarf sample. The kinematics are successfully predicted. The predicted metallicity distribution has too few stars with [Fe/H] ≃ -0.5 dex and too many metal-rich stars. A significant problem may be the lack of any chemical- kinematic correlations in our thick disc. We argue that EDFs will prove essential tools for the analysis of both observational data and sophisticated models of Galaxy formation and evolution.
@article{2015MNRAS.449.3479S, author = {{Sanders}, Jason L. and {Binney}, James}, title = {{Extended distribution functions for our Galaxy}}, journal = {\mnras}, keywords = {Galaxy: abundances, Galaxy: disc, Galaxy: evolution, Galaxy: kinematics and dynamics, solar neighbourhood, Astrophysics - Astrophysics of Galaxies}, year = {2015}, month = jun, volume = {449}, number = {4}, pages = {3479-3502}, doi = {10.1093/mnras/stv578}, archiveprefix = {arXiv}, eprint = {1501.02227}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2015MNRAS.449.3479S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
A fast algorithm for estimating actions in triaxial potentials

Jason L. Sanders, and James Binney

MNRAS, Mar 2015

Abs ADS arXiv pdf Bib DOI

We present an approach to approximating rapidly the actions in a general triaxial potential. The method is an extension of the axisymmetric approach presented by Binney, and operates by assuming that the true potential is locally sufficiently close to some Stäckel potential. The choice of Stäckel potential and associated ellipsoidal coordinates is tailored to each individual input phase-space point. We investigate the accuracy of the method when computing actions in a triaxial Navarro-Frenk-White potential. The speed of the algorithm comes at the expense of large errors in the actions, particularly for the box orbits. However, we show that the method can be used to recover the observables of triaxial systems from given distribution functions to sufficient accuracy for the Jeans equations to be satisfied. Consequently, such models could be used to build models of external galaxies as well as triaxial components of our own Galaxy. When more accurate actions are required, this procedure can be combined with torus mapping to produce a fast convergent scheme for action estimation.
@article{2015MNRAS.447.2479S, author = {{Sanders}, Jason L. and {Binney}, James}, title = {{A fast algorithm for estimating actions in triaxial potentials}}, journal = {\mnras}, keywords = {methods: numerical, Galaxy: kinematics and dynamics, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2015}, month = mar, volume = {447}, number = {3}, pages = {2479-2496}, doi = {10.1093/mnras/stu2598}, archiveprefix = {arXiv}, eprint = {1412.2093}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2015MNRAS.447.2479S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2014

MNRAS

Probabilistic model for constraining the Galactic potential using tidal streams

Jason L. Sanders

MNRAS, Sep 2014

Abs ADS arXiv pdf Bib DOI

We present a generative probabilistic model for a tidal stream and demonstrate how this model is used to constrain the Galactic potential. The model takes advantage of the simple structure of a stream in angle and frequency space for the correct potential. We investigate how the method performs on full 6D mock stream data, and mock data with outliers included. As currently formulated, the technique is computationally costly when applied to data with large observational errors, but we describe several modifications that promise to make the technique computationally tractable.

@article{2014MNRAS.443..423S,
  author = {{Sanders}, Jason L.},
  title = {{Probabilistic model for constraining the Galactic potential using tidal streams}},
  journal = {\mnras},
  keywords = {methods: numerical, Galaxy: kinematics and dynamics, Galaxy: structure, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies},
  year = {2014},
  month = sep,
  volume = {443},
  number = {1},
  pages = {423-431},
  doi = {10.1093/mnras/stu1159},
  archiveprefix = {arXiv},
  eprint = {1401.7602},
  primaryclass = {astro-ph.GA},
  adsurl = {https://ui.adsabs.harvard.edu/abs/2014MNRAS.443..423S},
  adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

MNRAS
Actions, angles and frequencies for numerically integrated orbits

Jason L. Sanders, and James Binney

MNRAS, Jul 2014

Abs ADS arXiv pdf Bib DOI

We present a method for extracting actions, angles and frequencies from an orbit’s time series. The method recovers the generating function that maps an analytic phase-space torus to the torus to which the orbit is confined by simultaneously solving the constraints provided by each time step. We test the method by recovering the actions and frequencies of tori in a triaxial Stäckel potential, and use it to investigate the structure of orbits in a triaxial potential that has been fitted to our Galaxy’s Sagittarius stream. The method promises to be useful for analysing N-body simulations. It also takes a step towards constructing distribution functions for the triaxial components of our Galaxy, such as the bar and dark halo.
@article{2014MNRAS.441.3284S, author = {{Sanders}, Jason L. and {Binney}, James}, title = {{Actions, angles and frequencies for numerically integrated orbits}}, journal = {\mnras}, keywords = {methods: numerical, Galaxy: kinematics and dynamics, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2014}, month = jul, volume = {441}, number = {4}, pages = {3284-3295}, doi = {10.1093/mnras/stu796}, archiveprefix = {arXiv}, eprint = {1401.3600}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2014MNRAS.441.3284S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2013

MNRAS
Stream-orbit misalignment - II. A new algorithm to constrain the Galactic potential

Jason L. Sanders, and James Binney

MNRAS, Aug 2013

Abs ADS arXiv pdf Bib DOI

In the first of these two papers, we demonstrated that assuming streams delineate orbits can lead to order one errors in potential parameters for realistic Galactic potentials. Motivated by the need for an improvement on orbit-fitting, we now present an algorithm for constraining the Galactic potential using tidal streams without assuming that streams delineate orbits. This approach is independent of the progenitor mass so is valid for all observed tidal streams. The method makes heavy use of angle- action variables and seeks the potential which recovers the expected correlations in angle space. We demonstrate that the method can correctly recover the parameters of a simple two- parameter logarithmic potential by analysing an N-body simulation of a stream. We investigate the magnitude of the errors in observational data for which the method can still recover the correct potential and compare this to current and future errors in data. The errors in the observables of individual stars for current and near future data are shown to be too large for the direct use of this method, but when the data are averaged in bins on the sky, the resulting averaged data are accurate enough to constrain correctly the potential parameters for achievable observational errors. From pseudo-data with errors comparable to those that will be furnished in the era of Gaia (20 per cent distance errors, 1.2 mas yr\(^-1\) proper motion errors, and 10 km s\(^-1\) line-of-sight velocity errors) we recover the circular velocity, V\(_c\) = 220 km s\(^- 1\) , and the flattening of the potential, q = 0.9, to be V\(_c\) = 223 \(\pm\) 10 km s\(^- 1\) and q = 0.91 \(\pm\) 0.09.
@article{2013MNRAS.433.1826S, author = {{Sanders}, Jason L. and {Binney}, James}, title = {{Stream-orbit misalignment - II. A new algorithm to constrain the Galactic potential}}, journal = {\mnras}, keywords = {methods: numerical, Galaxy: halo, Galaxy: kinematics and dynamics, Galaxy: structure, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2013}, month = aug, volume = {433}, number = {3}, pages = {1826-1836}, doi = {10.1093/mnras/stt816}, archiveprefix = {arXiv}, eprint = {1305.1937}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2013MNRAS.433.1826S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }
MNRAS
Stream-orbit misalignment - I. The dangers of orbit-fitting

Jason L. Sanders, and James Binney

MNRAS, Aug 2013

Abs ADS arXiv pdf Bib DOI

Tidal streams do not, in general, delineate orbits. A stream-orbit misalignment is expected to lead to biases when using orbit- fitting to constrain models for the Galactic potential. In this first of two papers, we discuss the expected magnitude of the misalignment and the resulting dangers of using orbit-fitting algorithms to constrain the potential. We summarize data for known streams which should prove useful for constraining the Galactic potential, and compute their actions in a realistic Galactic potential. We go on to discuss the formation of tidal streams in angle-action space, and explain why, in general, streams do not delineate orbits. The magnitude of the stream- orbit misalignment is quantified for a logarithmic potential and a multicomponent Galactic potential. Specifically, we focus on the expected misalignment for the known streams. By introducing a two-parameter family of realistic Galactic potentials we demonstrate that assuming that these streams delineate orbits can lead to order one errors in the halo flattening and halo-to- disc force ratio at the Sun. We present a discussion of the dependence of these results on the progenitor mass and demonstrate that the misalignment is mass independent for the range of masses of observed streams. Hence, orbit-fitting does not yield better constraints on the potential if one uses narrower, lower mass streams.
@article{2013MNRAS.433.1813S, author = {{Sanders}, Jason L. and {Binney}, James}, title = {{Stream-orbit misalignment - I. The dangers of orbit-fitting}}, journal = {\mnras}, keywords = {Galaxy: halo, Galaxy: kinematics and dynamics, Galaxy: structure, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies}, year = {2013}, month = aug, volume = {433}, number = {3}, pages = {1813-1825}, doi = {10.1093/mnras/stt806}, archiveprefix = {arXiv}, eprint = {1305.1935}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2013MNRAS.433.1813S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }

2012

MNRAS

Angle-action estimation in a general axisymmetric potential

Jason Sanders

MNRAS, Oct 2012

Abs ADS arXiv pdf Bib DOI

The usefulness of angle-action variables in galaxy dynamics is well known, but their use is limited due to the difficulty of their calculation in realistic galaxy potentials. Here we present a method for estimating angle-action variables in a realistic Milky Way axisymmetric potential by locally fitting a Stäckel potential over the region an orbit probes. The quality of the method is assessed by comparison with other known methods for estimating angle-action variables of a range of disc and halo-type orbits. We conclude by projecting the Geneva- Copenhagen survey into angle-action space.

@article{2012MNRAS.426..128S,
  author = {{Sanders}, Jason},
  title = {{Angle-action estimation in a general axisymmetric potential}},
  journal = {\mnras},
  keywords = {methods: numerical, Galaxy: kinematics and dynamics, solar neighbourhood, Galaxy: structure, galaxies: kinematics and dynamics, Astrophysics - Astrophysics of Galaxies},
  year = {2012},
  month = oct,
  volume = {426},
  number = {1},
  pages = {128-139},
  doi = {10.1111/j.1365-2966.2012.21698.x},
  archiveprefix = {arXiv},
  eprint = {1208.2813},
  primaryclass = {astro-ph.GA},
  adsurl = {https://ui.adsabs.harvard.edu/abs/2012MNRAS.426..128S},
  adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

MNRAS
Determining the velocity dispersion of the thick disc

Jason Sanders

MNRAS, Sep 2012

Abs ADS arXiv pdf Bib DOI

We attempt to recover the mean vertical velocity and vertical velocity dispersion as a function of the Galactic height for a sample drawn from a realistic Galaxy distribution function by following the method presented by Moni Bidin, Carraro & Mendez. We find that, for the sample size used, the observational error in the velocities is much smaller than the Poisson noise which has not been accounted for by Moni Bidin et al. We repeat the analysis on a large number of samples to estimate the contribution of the Poisson noise and to uncover any systematics. We find that the dispersion is systematically overestimated at low Galactic heights and slightly underestimated at high Galactic heights, leading to an underestimate of the gradient of the dispersion with Galactic height. The causes of the systematics are revealed by repeating the calculation using a method inspired by Girard et al. This method recovers the expected dispersion much more successfully and in particular yields a gradient of the dispersion with Galactic height which is approximately three times that found using the method presented by Moni Bidin et al.
@article{2012MNRAS.425.2228S, author = {{Sanders}, Jason}, title = {{Determining the velocity dispersion of the thick disc}}, journal = {\mnras}, keywords = {methods: data analysis, methods: numerical, Galaxy: kinematics and dynamics, solar neighbourhood, Astrophysics - Astrophysics of Galaxies}, year = {2012}, month = sep, volume = {425}, number = {3}, pages = {2228-2233}, doi = {10.1111/j.1365-2966.2012.21619.x}, archiveprefix = {arXiv}, eprint = {1205.5397}, primaryclass = {astro-ph.GA}, adsurl = {https://ui.adsabs.harvard.edu/abs/2012MNRAS.425.2228S}, adsnote = {Provided by the SAO/NASA Astrophysics Data System} }