http://arxiv.org/abs/1902.05268
The velocity distribution of stars is a sensitive probe of the gravitational potential of the Galaxy, and hence of its dark matter distribution. In particular, the shape of the dark halo (e.g. spherical, oblate, prolate) determines velocity correlations, and different halo geometries are expected to result in measurable differences. We here explore and interpret the correlations in the $(v_R, v_z)$-velocity distribution as a function of position in the Milky Way. We select a high quality sample of stars from the Gaia DR2 catalog, and characterise the orientation of the velocity distribution or tilt angle, over a radial distance range of $[3-13]$ kpc and up to $4$ kpc away from the Galactic plane while taking into account the effects of the measurement errors. The velocity ellipsoid is consistent with spherical alignment at $R\sim4$~kpc, while it progressively becomes shallower at larger Galactocentric distances and is cylindrically aligned at $R=12$~kpc for all heights probed. Although the systematic parallax errors present in Gaia DR2 likely impact our estimates of the tilt angle at large Galactocentric radii, possibly making it less spherically aligned, their amplitude is not enough to explain the deviations from spherical alignment. We find that the tilt angles do not strongly vary with Galactic azimuth and that different stellar populations depict similar tilt angles. We introduce a simple analytic function that describes well these trends over the full radial range explored.
J. Hagen, A. Helmi, P. Zeeuw, et. al.
Fri, 15 Feb 19
9/48
Comments: 13 pages, 12 figures, submitted to A&A
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