Ratios of star cluster core and half-mass radii: a cautionary note on intermediate-mass black holes in star clusters

Abstract

There is currently much interest in the possible presence of intermediate-mass black holes in the cores of globular clusters. Based on theoretical arguments and simulation results it has previously been suggested that a large core radius -- or particularly a large ratio of the core radius to half-mass radius -- is a promising indicator for finding such a black hole in a star cluster. In this study N-body models of 100000 stars with and without primordial binaries are used to investigate the long-term structural evolution of star clusters. Importantly, the simulation data is analysed using the same processes by which structural parameters are extracted from observed star clusters. This gives a ratio of the core and half-mass (or half-light) radii that is directly comparable to the Galactic globular cluster sample. As a result, it is shown that the ratios observed for the bulk of this sample can be explained without the need for an intermediate-mass black hole. Furthermore, it is possible that clusters with large core to half-light radius ratios harbour a black-hole binary (comprised of stellar mass black holes) rather than a single massive black hole. This work does not rule out the existence of intermediate-mass black holes in the cores of at least some star clusters.Comment: 14 pages, 7 figures, accepted for publication in MNRA