Evidence for two populations of Galactic globular clusters from the ratio of their half-mass to Jacobi radii

Abstract

We investigate the ratio between the half-mass radii rh of Galactic globular clusters and their Jacobi radii rJ given by the potential of the Milky Way and show that clusters with galactocentric distances RGC > 8 kpc fall into two distinct groups: one group of compact, tidally underfilling clusters with rh/rJ < 0.05 and another group of tidally filling clusters which have 0.1 < 0.3. We find no correlation between the membership of a particular cluster in one of these groups and its membership in the old or younger halo population. Based on the relaxation times and orbits of the clusters, we argue that compact clusters and most clusters in the inner Milky Way were born compact with half-mass radii rh < 1 pc. Some of the tidally filling clusters might have formed compact as well, but the majority likely formed with large half-mass radii. Galactic globular clusters therefore show a similar dichotomy as was recently found for globular clusters in dwarf galaxies and for young star clusters in the Milky Way. It seems likely that some of the tidally filling clusters are evolving along the main-sequence line of clusters recently discovered by Küpper et al. (2008) and are in the process of dissolution. © 2009 RAS.