The gravitational force field of the Galaxy measured from the kinematics of RR Lyrae in Gaia

Abstract

From a sample of 15651 RR Lyrae with accurate proper motions in Gaia DR2, we measure the azimuthally averaged kinematics of the inner stellar halo between 1.5 kpc and 20 kpc from the Galactic centre. We find that their kinematics are strongly radially anisotropic, and their velocity ellipsoid nearly spherically aligned over this volume. Only in the inner regions 5 kpc does the anisotropy significantly fall (but still with β > 0.25) and the velocity ellipsoid tilt towards cylindrical alignment. In the inner regions, our sample of halo stars rotates at up to 50 km s −1 , which may reflect the early history of the Milky Way, although there is also significant angular momentum exchange with the Galactic bar at these radii. We subsequently apply the Jeans equations to these kinematic measurements in order to non-parametrically infer the azimuthally averaged gravitational acceleration field over this volume, and by removing the contribution from baryonic matter, measure the contribution from dark matter. We find that the gravitational potential of the dark matter is nearly spherical with average flattening q Φ = 1.01 ± 0.06 between 5 kpc and 20 kpc, and by fitting parametric ellipsoidal density profiles to the acceleration field, we measure the flattening of the dark matter halo over these radii to be q ρ = 1.00 ± 0.09.

The biggest splash

“…Deason et al (2018) connect the orbital properties of the local Main Sequence stars and the distant Blue Horizontal Branch stars and show that the two samples have very similar apo-centric distance, thus establishing that the local and the far-field halo tracers likely share a common progenitor. Using independent sets of halo tracers, Wegg et al (2019) and Tian et al (2019) confirm that the GS debris has a small but statistically significant prograde rotation in agreement with Deason et al (2017); Belokurov et al (2018); Myeong et al (2018b). Myeong et al (2019) present chemical and kinematic evidence for two (perhaps related) accretion events: the nearly radial GS merger (see Belokurov et al 2018) and the highly retrograde Sequoia (see also Myeong et al 2018a;Matsuno et al 2019).…”

Velocity substructure from Gaia and direct searches for dark matter

“…Although these cannot be determined from Gaia data, there are some general statements that we can make about their relative values. For example, the gravitational potential is nearly spherical [110], so the DM associated with the Sausage and the Shards is subdominant. In Ref.…”

The tale of the tail – disentangling the high transverse velocity stars in Gaia DR2

“…Since Belokurov et al (2018) only explore the halo for z > 1 kpc, we have extrapolated their results and assumed that the features observed for 1 < z < 3 kpc are the same for z < 1 kpc. Note that the tilt of the halo ellipsoid is likely to be negligible for our sample (see, for example, Wegg et al 2019) and so we are safe to neglect any correlations in the velocities.…”