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 clusters with
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 to 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 Kuepper et al. (2008) and are in the process of
dissolution.
