Collisionless dynamics in globular clusters

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 fitted by King models. However, for many GCs, especially those with Hubble Space Telescope-resolved central regions and 'extratidal' features, King models provide poor fits. We suggest that this is partly because collisionless dynamics is also important and contributes to shaping the cluster properties. We show using time-scale and length-scale arguments that except for the very centres of clusters, collisionless dynamics should be more important than collisional. We then fit 38 GCs analysed by Noyola & Gebhardt 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 approximately five times better in terms 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.).