Collisionless dynamics in Globular Clusters

TL;DR

This study challenges the traditional view that globular cluster dynamics are purely collisional, demonstrating that collisionless models like DARKexp better fit observed data for many clusters, especially outside their centers.

Contribution

The paper introduces the application of collisionless DARKexp models to globular clusters, showing they often outperform King models in fitting observational data.

Findings

DARKexp models fit 29 out of 38 GCs better than King models.

Collisionless dynamics are significant outside the very centers of GCs.

DARKexp models are based on maximum entropy principles for collisionless systems.

Abstract

Since globular clusters (GCs) are old, low-N systems their dynamics is widely believed to be fully dominated by collisional two-body processes, and their surface brightness profiles are fit by King models. However, for many GCs, especially those with HST-resolved central regions, and `extra-tidal' features, King models provide poor fits. We suggest that this is partly because collisionless dynamics is also important and contribute to shaping the cluster properties. We show using time-scale and length-scale arguments that except for the very centers of clusters, collisionless dynamics should be more important than collisional. We then fit 38 GCs analyzed by Noyola and Gebhardt (2006) with (collisional) King and (collisionless) DARKexp models over the full available radial range, and find that the latter provide a better fit to 29 GCs; for six of these the fit is at least ~5x better in term of rms. DARKexp models are theoretically derived maximum entropy equilibrium states of self-gravitating collisionless systems and have already been shown to fit the results of dark matter N-body simulations. (We do not attempt fits with ad hoc fitting functions.)