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

TL;DR

This study reveals two distinct populations of Galactic globular clusters based on their size ratios relative to the Jacobi radius, indicating different formation and evolutionary histories.

Contribution

It demonstrates the existence of two populations of globular clusters distinguished by their size ratios and suggests different origins and evolutionary paths for these groups.

Findings

Clusters with R_{GC}>8 kpc fall into two groups based on r_h/r_J ratios.

No correlation between cluster group membership and halo population age.

Inner clusters likely formed compact, while many outer clusters formed with larger sizes.

Abstract

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.