Which globular clusters contain intermediate-mass black holes?

Abstract

It has been assumed that intermediate-mass black holes (IMBHs) in globular clusters can only reside in the most centrally concentrated clusters, with a so-called core-collapsed density profile. While this would be a natural guess, it is in fact wrong. We have followed the evolution of star clusters containing IMBHs with masses between 125 less than or equal to M-BH less than or equal to 1000 M-circle dot through detailed N-body simulations, and we find that a cluster with an IMBH, in projection, appears to have a relatively large "core'' with surface brightness only slightly rising toward the center. This makes it highly unlikely that any of the "core-collapsed'' clusters will harbor an IMBH. On the contrary, the places to look for an IMBH are those clusters that can be fitted well by medium-concentration King models. The velocity dispersion of the visible stars in a globular cluster with an IMBH is nearly constant well inside the apparent core radius. For a cluster of mass M-C containing an IMBH of mass M-BH, the influence of the IMBH becomes significant only at a fraction 2.5M(BH)/M-C of the half-mass radius, deep within the core, where it will affect only a small number of stars. In conclusion, observational detection of an IMBH may be possible, but will be challenging