Multiple stellar-mass black holes in globular clusters: theoretical confirmation

While tens or hundreds of stellar-remnant black holes (BHs) are expected to form in globular star clusters, it is still unclear how many of those will be retained upon formation, and how many will be ejected through subsequent dynamical interactions. No such BHs have been found in any Milky Way globular cluster until the recent discovery of stellar-mass BHs in the globular cluster M22 (NGC 6656) with now an estimated population of 5-100 BHs. We present a direct N-body model of a star cluster of the same absolute and dynamical age as M22. Imposing an initial retention fraction of approximate to 10 per cent for BHs, 16 stellar-remnant BHs are retained at a cluster age of 12 Gyr, in agreement with the estimate for M22. Of those 16 BHs, two are in a binary system with a main-sequence star each while also one pure BH binary is present. We argue that multiple BHs can be present in any Milky Way cluster with an extended core radius, such as M22 or the model presented here.