Stellar Velocity Dispersion and Anisotropy of the Milky Way Inner Halo
  We measure the three components of velocity dispersion,
$\sigma_{R},\sigma_{\theta},\sigma_{\phi}$, for stars within 6 < R < 30 kpc of
the Milky Way using a new radial velocity sample from the MMT telescope. We
combine our measurements with previously published data so that we can more
finely sample the stellar halo. We use a maximum likelihood statistical method
for estimating mean velocities, dispersions, and covariances assuming only that
velocities are normally distributed. The alignment of the velocity ellipsoid is
consistent with a spherically symmetric gravitational potential. From the
spherical Jeans equation, the mass of the Milky Way is M(<12 kpc) =
$1.3\times10^{11}$ M$_{\odot}$ with an uncertainty of 40%. We also find a
region of discontinuity, 15 < R < 25 kpc, where the estimated velocity
dispersions and anisotropies diverge from their anticipated values, confirming
at high significance the break observed by others. We argue that this break in
anisotropy is physically explained by coherent stellar velocity structure in
the halo, such as the Sgr stream. To significantly improve our understanding of
halo kinematics will require combining radial velocities with future Gaia
proper motions.
