Absolute Hubble Space Telescope Proper Motion (HSTPROMO) of Distant Milky Way Globular Clusters: Galactocentric Space Velocities and the Milky Way Mass

Abstract

We present Hubble Space Telescope ( HST ) absolute proper motion (PM) measurements for 20 globular clusters (GCs) in the Milky Way (MW) halo at Galactocentric distances R GC ≈ 10 – 100 kpc, with a median per-coordinate PM uncertainty of 0.06 mas yr − 1 . Young and old halo GCs do not show systematic differences in their 3D Galactocentric velocities, derived from combining existing line-of-sight velocities. We confirm the association of Arp 2, Pal 12, Terzan 7, and Terzan 8 with Sgr. These clusters and NGC 6101 have tangential velocity v tan > 290 km s −1 , whereas all other clusters have v tan  < 200 km s −1 . NGC 2419, the most distant GC in our sample, is also likely associated with the Sgr stream, whereas NGC 4147, NGC 5024, and NGC 5053 definitely are not. We use the distribution of orbital parameters derived using the 3D velocities to separate halo GCs that either formed within the MW or were accreted. We also assess the specific formation history of, e.g., Pyxis and Terzan 8. We constrain the MW mass via an estimator that considers the full 6D phase-space information for 16 of the GCs from R GC = 10 to 40 kpc. The velocity dispersion anisotropy parameter β = 0.609 − 0.229 + 0.130 . The enclosed mass M ( < 39.5 kpc ) = 0.61 − 0.12 + 0.18 × 10 12 M ⊙ , and the virial mass M vir = 2.05 − 0.79 + 0.97 × 10 12 M ⊙ . These are consistent with, but on the high side among, recent mass estimates in the literature.