The velocity anisotropy of the Milky Way satellite system
  We analyse the orbital kinematics of the Milky Way (MW) satellite system
utilizing the latest systemic proper motions for 38 satellites based on data
from Gaia Data Release 2. Combining these data with distance and line-of-sight
velocity measurements from the literature, we use a likelihood method to model
the velocity anisotropy, $\beta$, as a function of Galactocentric distance and
compare the MW satellite system with those of simulated MW-mass haloes from the
APOSTLE and Auriga simulation suites. The anisotropy profile for the MW
satellite system increases from $\beta\sim -2$ at $r\sim20$ kpc to $\beta\sim
0.5$ at $r\sim200$ kpc, indicating that satellites closer to the Galactic
centre have tangentially-biased motions while those farther out have
radially-biased motions. The motions of satellites around APOSTLE host galaxies
are nearly isotropic at all radii, while the $\beta(r)$ profiles for satellite
systems in the Auriga suite, whose host galaxies are substantially more massive
in baryons than those in APOSTLE, are more consistent with that of the MW
satellite system. This shape of the $\beta(r)$ profile may be attributed to the
central stellar disc preferentially destroying satellites on radial orbits, or
intrinsic processes from the formation of the Milky Way system.
