The tilt of the velocity ellipsoid in the Milky Way with Gaia DR2. (arXiv:1902.05268v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hagen_J/0/1/0/all/0/1">Jorrit H. J. Hagen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Helmi_A/0/1/0/all/0/1">Amina Helmi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zeeuw_P/0/1/0/all/0/1">P. Tim de Zeeuw</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Posti_L/0/1/0/all/0/1">Lorenzo Posti</a>
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 $Rsim4$~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.
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 $Rsim4$~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.
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