Retention of Stellar-Mass Black Holes in Globular Clusters
  Globular clusters should be born with significant numbers of stellar-mass
black holes (BHs). It has been thought for two decades that very few of these
BHs could be retained through the cluster lifetime. With masses ~10 MSun, BHs
are ~20 times more massive than an average cluster star. They segregate into
the cluster core, where they may eventually decouple from the remainder of the
cluster. The small-N core then evaporates on a short timescale. This is the
so-called Spitzer instability. Here we present the results of a full dynamical
simulation of a globular cluster containing many stellar-mass BHs with a
realistic mass spectrum. Our Monte Carlo simulation code includes detailed
treatments of all relevant stellar evolution and dynamical processes. Our main
finding is that old globular clusters could still contain many BHs at present.
In our simulation, we find no evidence for the Spitzer instability. Instead,
most of the BHs remain well-mixed with the rest of the cluster, with only the
innermost few tens of BHs segregating significantly. Over the 12 Gyr evolution,
fewer than half of the BHs are dynamically ejected through strong binary
interactions in the cluster core. The presence of BHs leads to long-term
heating of the cluster, ultimately producing a core radius on the high end of
the distribution for Milky Way globular clusters (and those of other galaxies).
A crude extrapolation from our model suggests that the BH--BH merger rate from
globular clusters could be comparable to the rate in the field.
