Globular Cluster Scale Sizes in Giant Galaxies: The Case of M87 and the Role of Orbital Anisotropy and Tidal Filling

Abstract

We present new Hubble Space Telescope imaging of the outer regions of M87 in order to study its globular cluster (GC) population out to large galactocentric distances. We discuss, particularly, the relationship between GC effective radii r_h and projected galactocentric distance R _gc. The observations suggest a shallow trend r_h \propto R_{gc}^{0.14} out to R _gc ~ 100 kpc, in agreement with studies of other giant elliptical galaxies. To theoretically reproduce this relationship, we simulate GC populations with various distributions of orbits. For an isotropic distribution of cluster orbits, we find a steeper trend of r_h \propto R_{gc}^{0.4}. Instead, we suggest that (1) if the cluster system has an orbital anisotropy profile, where orbits become preferentially radial with increasing galactocentric distance, and (2) if clusters become more tidally underfilling with galactocentric distance, then the observed relationship can be recovered. We also apply this approach to the red and blue GC populations separately and predict that red clusters are preferentially underfilling at large R _gc and have a more isotropic distribution of orbits than blue clusters.