milky way chemo-dynamical disc modelling

2020

1. 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

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   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}
   }

2019

1. 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}
   }

2. ApJ

   The Inside-out Growth of the Galactic Disk

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

   ApJ, Oct 2019

   Abs ADS arXiv pdf Bib DOI

   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}
   }

2018

1. 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}
   }

2. ApJL

   The Milky Way Halo in Action Space

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

   ApJL, Apr 2018

   Abs ADS arXiv pdf Bib DOI

   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}
   }

2015

1. MNRAS

   Extended distribution functions for our Galaxy

   Jason L. Sanders, and James Binney

   MNRAS, Jun 2015

   Abs ADS arXiv pdf Bib DOI

   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}
   }