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

Abstract

We employ Gaia DR2 proper motions for 151 Milky Way globular clusters from Vasiliev (2019) 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. (2018) who distinguished among different Galactic stellar components using stars's orbits. In particular, we use the ratio $L_z/e$, the $Z$ projection of the angular momentum to the eccentricity, as population tracer, which we complement with chemical abundances extracted from the literature and Monte-Carlo simulations. We find that 20 globular clusters 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 globular clusters have close to zero rotational velocity with average value $<\Theta >$ =1$\pm$ 4 km s${}^{-1}$. On the other hand, the sample of clusters that belong to the thick disk possesses a significant rotation with average rotational velocity 179 $\pm$ 6 km s${}^{-1}$. The twenty globular clusters orbiting within the bar/bulge region of the Milky Way galaxy have average rotational velocity of 49 $\pm$ 11 km s${}^{-1}$.

arXiv comment25 pages, 10 Figures, 5 Tables. Accepted to Astrophysical Journal

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