Dynamics of stellar black holes in young star clusters with different
  metallicities - II. Black hole-black hole binaries
  In this paper, we study the formation and dynamical evolution of black
hole-black hole (BH-BH) binaries in young star clusters (YSCs), by means of
N-body simulations. The simulations include metallicity-dependent recipes for
stellar evolution and stellar winds, and have been run for three different
metallicities (Z = 0.01, 0.1 and 1 Zsun). Following recent theoretical models
of wind mass-loss and core-collapse supernovae, we assume that the mass of the
stellar remnants depends on the metallicity of the progenitor stars. We find
that BH-BH binaries form efficiently because of dynamical exchanges: in our
simulations, we find about 10 times more BH-BH binaries than double neutron
star binaries. The simulated BH-BH binaries form earlier in metal-poor YSCs,
which host more massive black holes (BHs) than in metal-rich YSCs. The
simulated BH-BH binaries have very large chirp masses (up to 80 Msun), because
the BH mass is assumed to depend on metallicity, and because BHs can grow in
mass due to the merger with stars. The simulated BH-BH binaries span a wide
range of orbital periods (10^-3-10^7 yr), and only a small fraction of them
(0.3 per cent) is expected to merge within a Hubble time. We discuss the
estimated merger rate from our simulations and the implications for Advanced
VIRGO and LIGO.
