2019ApJ...873..118W

We estimate the mass of the Milky Way (MW) within 21.1 kpc using the kinematics of halo globular clusters (GCs) determined by Gaia. The second Gaia data release (DR2) contained a catalog of absolute proper motions (PMs) for a set of Galactic GCs and satellite galaxies. We select from the catalog only halo GCs, identifying a total of 34 GCs spanning 2.0≤ r≤ 21.1kpc, and use their 3D kinematics to estimate the anisotropy over this range to be β=0.46_–0.19^+0.15, in good agreement with, though slightly lower than, a recent estimate for a sample of halo GCs using Hubble Space Telescope (HST) PM measurements further out in the halo. We then use the Gaia kinematics to estimate the mass of the MW inside the outermost GC to be M(< 21.1kpc)=0.21_–0.03^+0.04×10¹²M_☉, which corresponds to a circular velocity at r_{max} of v_circ(21.1kpc)=206_–16⁺¹⁹kms^–1. The implied virial mass is M_virial=1.28_–0.48^+0.97×10¹²M_☉. The error bars encompass the uncertainties on the anisotropy and on the density profile of the MW dark halo, and the scatter inherent in the mass estimator we use. We get improved estimates when we combine the Gaia and HST samples to provide kinematics for 46 GCs out to 39.5 kpc: β=0.52_–0.14^+0.11, M(< 39.5kpc)=0.42_–0.06^+0.07×10¹²M_☉, and M_virial=1.54_–0.44^+0.75×10¹²M_☉. We show that these results are robust to potential substructure in the halo GC distribution. While a wide range of MW virial masses have been advocated in the literature, from below 10¹² M_☉ to above 2 x 10¹² M_☉, these new data imply that an intermediate mass is most likely.