Evidence for an Intermediate-mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
Abstract
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≤slant r≤slant 21.1 {kpc}, 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(< 21.1 {kpc})={0.21}-0.03+0.04× {10}12 {M}⊙ , which corresponds to a circular velocity at {r}\max of {v}circ}(21.1 {kpc})={206}-16+19 {km} {{{s}}}-1. The implied virial mass is {M}virial}={1.28}-0.48+0.97× {10}12 {M}⊙ . 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(< 39.5 {kpc})={0.42}-0.06+0.07× {10}12 {M}⊙ , and {M}virial}={1.54}-0.44+0.75× {10}12 {M}⊙ . 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 × 1012 {M}⊙ , these new data imply that an intermediate mass is most likely.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 2019
- DOI:
- 10.3847/1538-4357/ab089f
- arXiv:
- arXiv:1804.11348
- Bibcode:
- 2019ApJ...873..118W
- Keywords:
-
- dark matter;
- Galaxy: fundamental parameters;
- Galaxy: halo;
- Galaxy: kinematics and dynamics;
- Galaxy: structure;
- globular clusters: general;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 16 pages, 9 figures, 3 tables, accepted for publication in ApJ (updated to match accepted version)
