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Evidence for an Intermediate-mass Milky Way from Gaia DR2 Halo Globular Cluster Motions

Accepted version
Peer-reviewed

Type

Article

Change log

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 catalogue of absolute proper motions (PMs) for a set of Galactic GCs and satellite galaxies measured using Gaia DR2 data. We select from the catalogue only halo GCs, identifying a total of 34 GCs spanning 2.0<r<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, though slightly lower than, a recent estimate for a sample of halo GCs using 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+0.041012M, which corresponds to a circular velocity of vcirc(21.1kpc)=206−16+19 km/s. The implied virial mass is Mvirial=1.28−0.48+0.971012M. 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+0.071012M, and Mvirial=1.54−0.44+0.751012M. 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 1012M to above 2×1012M, these new data imply that an intermediate mass is most likely.

Description

Keywords

dark matter, Galaxy: fundamental parameters, Galaxy: halo, Galaxy: kinematics and dynamics, Galaxy: structure, globular clusters: general

Journal Title

Astrophysical Journal

Conference Name

Journal ISSN

0004-637X
1538-4357

Volume Title

873

Publisher

American Astronomical Society
Sponsorship
Science and Technology Facilities Council (ST/N000927/1)