The tilt of the velocity ellipsoid in the Milky Way with Gaia DR2

Abstract

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, or prolate) determines velocity correlations, and different halo geometries are expected to result in measurable differences. Here we explore and interpret the correlations in the (v R , v z )-velocity distribution as a function of position in the Milky Way. We selected a high-quality sample of stars from the Gaia DR2 catalogue and characterised the orientation of the velocity distribution or tilt angle over a radial distance range of [4 − 13] kpc and up to 3.5 kpc away from the Galactic plane while taking into account the effects of the measurement errors. We find that the tilt angles change from spherical alignment in the inner Galaxy (R ∼ 4 kpc) towards more cylindrical alignments in the outer Galaxy (R ∼ 11 kpc) when using distances that take a global zero-point offset in the parallax of −29µas. However, if the amplitude of this offset is underestimated, then the inferred tilt angles in the outer Galaxy only appear shallower and are intrinsically more consistent with spherical alignment for an offset as large as −54µas. We further find that the tilt angles do not seem to strongly vary with Galactic azimuth and that different stellar populations depict similar tilt angles. Therefore we introduce a simple analytic function that describes the trends found over the full radial range. Since the systematic parallax errors in Gaia DR2 depend on celestial position, magnitude, and colour in complex ways, it is not possible to fully correct for them. Therefore it will be particularly important for dynamical modelling of the Milky Way to thoroughly characterise the systematics in astrometry in future Gaia data releases.

Dynamical Model of the Milky Way Using APOGEE and Gaia Data

“…As a dynamical model, we use the JAM sph approach 7 (Cappellari 2020), which is based on the solution of the axisymmetric Jeans equations and assumes a spherically aligned velocity ellipsoid (Bacon et al 1983;Bacon 1985). The spherical alignment has proven to describe the Gaia data in the outer halo (Wegg et al 2019) and in the disk region (Hagen et al 2019;Everall et al 2019). This was also confirmed by the results in Paper I.…”

“…Previously, we used the Gaia Data Release 2 (DR2) kinematics to construct an axisymmetric dynamical model of the Milky Way disk (Nitschai et al 2020, hereafter Paper I). There we used the new spherically aligned Jeans anisotropic modeling (JAM sph ; Cappellari 2020) method, since the Gaia data (Everall et al 2019;Hagen et al 2019) showed that the velocity ellipsoid is closer to being spherically aligned than cylindrically (JAM cyl ;Cappellari 2008). But we also compared the results against JAM cyl and found negligible differences.…”

First Gaia dynamical model of the Milky Way disc with six phase space coordinates: a test for galaxy dynamics

“…As an extreme test of the sensitivity of our model to the assumptions on the orientation of the velocity ellipsoid, we also have fit a model (JAM cyl ) which assumes a cylindrically-aligned velocity ellipsoid (Cappellari 2008). The JAM sph model gives a slightly better fit the data than JAM sph , consistently with the finding that the Milky Way velocity ellipsoid is nearly spherical aligned (Hagen et al 2019;Everall et al 2019). However, the difference between the parameters inferred by two JAM sph and JAM cyl models is minimal as the two solutions are not very different in the disc plane.…”

General model of vertical distribution of stars in the Milky Way using complete Jeans equations

“…But its contribution has been neglected in most of the dynamical models so far (Bahcall 1984;Cappellari 2008) as is justified for a region very close to the mid-plane in a thin disc. Note that recent kinematic data from observations like RAVE, SDSS, and GAIA show that the stellar velocity ellipsoid is indeed tilted in the meridional plane both in the stellar disc (Hagen et al,2019;Evarall et al,2019) and in the outer stellar halo (Wegg et al,2019). It is also observed to have an orientation such that it tends to align with a spherical polar coordinate system centered at the center of the Galaxy.…”