Collisionless dynamics in Globular Clusters

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.)

27 pages, 18 figures; accepted to MNRAS

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Cited by in corpus (7)

• Star count density profiles and structural parameters of 26 Galactic globular clusters
• Globular cluster number density profiles using Gaia DR2
• Non-universality of dark-matter halos: cusps, cores, and the central potential
• Statistical mechanics of collisionless orbits. IV. Distribution of angular momentum
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• Statistical mechanics of self-gravitating systems: mixing as a criterion for indistinguishability
• Addressing the core-cusp and diversity problem of dwarf and disk galaxies using cold collisionless DARKexp theory