2012MNRAS.423.3589W

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.)