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

We investigate the ratio between the half-mass radii r(h) of Galactic globular clusters and their Jacobi radii r(J) given by the potential of the Milky Way and show that clusters with galactocentric distances R-GC > 8 kpc fall into two distinct groups: one group of compact, tidally underfilling clusterswith r(h)/r(J) < 0.05 and another group of tidally filling clusters which have 0.1 < r(h)/r(J) < 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 r(h) < 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 Kupper et al. (2008) and are in the process of dissolution.