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.)Comment: 27 pages, 18 figures; accepted to MNRA
