Mean proper motions, space orbits, and velocity dispersion profiles of Galactic globular clusters derived from<i>Gaia</i>DR2 data

Abstract

We have derived the mean proper motions and space velocities of 154 Galactic\nglobular clusters and the velocity dispersion profiles of 141 globular clusters\nbased on a combination of Gaia DR2 proper motions with ground-based\nline-of-sight velocities. Combining the velocity dispersion profiles derived\nhere with new measurements of the internal mass functions allows us to model\nthe internal kinematics of 144 clusters, more than 90% of the currently known\nGalactic globular cluster population. We also derive the initial cluster masses\nby calculating the cluster orbits backwards in time applying suitable recipes\nto account for mass loss and dynamical friction. We find a correlation between\nthe stellar mass function of a globular cluster and the amount of mass lost\nfrom the cluster, pointing to dynamical evolution as one of the mechanisms\nshaping the mass function of stars in clusters. The mass functions also show\nstrong evidence that globular clusters started with a bottom-light initial mass\nfunction. Our simulations show that the currently surviving globular cluster\npopulation has lost about 80% of its mass since the time of formation. If\nglobular clusters started from a log-normal mass function, we estimate that the\nMilky Way contained about 500 globular clusters initially, with a combined mass\nof about $2.5 \\cdot 10^8$ $M_\\odot$. For a power-law initial mass function, the\ninitial mass in globular clusters could have been a factor of three higher.\n