Proper motions and dynamics of the Milky Way globular cluster system from Gaia DR2

TL;DR

This paper uses Gaia DR2 data to analyze the motions and distribution of nearly all known Milky Way globular clusters, revealing their kinematic properties and implications for the Galaxy's gravitational potential.

Contribution

It provides the first comprehensive proper motion measurements for almost all Milky Way globular clusters and models their dynamics to infer the Galaxy's mass distribution.

Findings

Clusters within 10 kpc show significant rotation and isotropic velocity dispersion.

Outer clusters exhibit radially anisotropic orbits and high radial action concentration.

The Milky Way's circular velocity between 10-50 kpc is 210-240 km/s.

Abstract

We use Gaia Data Release 2 to determine the mean proper motions for 150 Milky Way globular clusters (almost the entire known population), with a typical uncertainty of 0.05 mas/yr limited mainly by systematic errors. Combining them with distance and line-of-sight velocity measurements from the literature, we analyze the distribution of globular clusters in the 6d phase space, using both position/velocity and action/angle coordinates. The population of clusters in the central 10 kpc has a mean rotational velocity reaching 50-80 km/s, and a nearly isotropic velocity dispersion 100-120 km/s, while in the outer galaxy the cluster orbits are strongly radially anisotropic. We confirm a concentration of clusters at high radial action in the outer region of the Galaxy. Finally, we explore a range of equilibrium distribution function-based models for the entire globular cluster system, and the information they provide about the potential of the Milky Way. The dynamics of clusters is best described by models with the circular velocity between 10 and 50 kpc staying in the range 210-240 km/s.