Multiple stellar-mass black holes in globular clusters: theoretical
  confirmation
  While tens or hundreds of stellar-remnant black holes 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 black holes have been found in any Milky Way globular
cluster until the recent discovery of stellar-mass black holes in the globular
cluster M22 (NGC 6656) with now an estimated population of 5-100 black holes.
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 approx. 10% for
black holes, 16 stellar-remnant black holes 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 black hole
binary is present. We argue that multiple black holes can be present in any
Milky Way cluster with an extended core radius, such as M22 or the model
presented here.
