The Gravitational Force Field of the Galaxy Measured From the Kinematics
  of RR Lyrae in Gaia
  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 $\lesssim$ 5 kpc does the
anisotropy significantly fall (but still with $\beta >$ 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 $q_\Phi=$ 1.01 $\pm$ 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 $q_\rho=$ 1.00 $\pm$ 0.09.
