Milky Way Subsystems from Globular Cluster Kinematics Using Gaia DR2 and HST Data

We employ Gaia DR2 proper motions for 151 Milky Way globular clusters (GCs) from Vasiliev in tandem with distances and line-of-sight velocities to derive their kinematical properties. To assign clusters to the Milky Way thick disk, bulge, and halo, we follow the approach of Posti et al., who distinguished among different Galactic stellar components using stars' orbits. In particular, we use the ratio L_z/e, the Z projection of the angular momentum to the eccentricity, as a population tracer, which we complement with chemical abundances extracted from the literature and Monte Carlo simulations. We find that 20 GCs belong to the bar/bulge of the Milky Way, 35 exhibit disk properties, and 96 are members of the halo. Moreover, we find that halo GCs have close to zero rotational velocity with an average value $\langle {\rm{\Theta }}\rangle =1\pm 4$ km s^-1. On the other hand, the sample of clusters that belong to the thick disk possess a significant rotation with average rotational velocity 179 ± 6 km s^-1. The 20 GCs orbiting within the bar/bulge region of the Milky Way have an average rotational velocity of 49 ± 11 km s^-1.