milky way bar

revealing the properties of the galaxy's bar

There is a bar at the centre of the Galaxy but it has not yet been mapped out in full detail. In particular, the number of stars for which we have accurate 3d velocities is low. Using a combination of the infra-red VVV survey fixed to an absolute reference frame with the Gaia data we have measured 2d velocities over the entire bar region. This can be converted into 3d kinematic maps like the one shown.

The rotation field inside the Milky Way's bar

Using this data and a novel technique, we have been able to measure the pattern speed of the bar from transverse velocity data for the first time.

(Sanders et al. 2019; Sanders et al. 2019; McGough et al. 2020)

References

2020

MNRAS
Models of bars - II. Exponential profiles

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

MNRAS, Apr 2020

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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} }

2019

MNRAS
The pattern speed of the Milky Way bar from transverse velocities

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

MNRAS, Oct 2019

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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} }
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

Abs ADS arXiv pdf Bib DOI

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} }