  We present N-body simulations of intermediate-mass (3000-4000 Msun) young
star clusters (SCs) with three different metallicities (Z=0.01, 0.1 and 1
Zsun), including metal-dependent stellar evolution recipes and binary
evolution. 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. In particular, massive metal-poor
stars (Z<=0.3 Zsun) are enabled to form massive stellar black holes (MSBHs,
with mass >=25 Msun) through direct collapse. We find that three-body
encounters, and especially dynamical exchanges, dominate the evolution of the
MSBHs formed in our simulations. In SCs with Z=0.01 and 0.1 Zsun, about 75 per
cent of simulated MSBHs form from single stars and become members of binaries
through dynamical exchanges in the first 100 Myr of the SC life. This is a
factor of >~3 more efficient than in the case of low-mass (<25 Msun) stellar
black holes. A small but non-negligible fraction of MSBHs power wind-accreting
(10-20 per cent) and Roche lobe overflow (RLO, 5-10 per cent) binary systems.
The vast majority of MSBH binaries that undergo wind accretion and/or RLO were
born from dynamical exchange. This result indicates that MSBHs can power X-ray
binaries in low-metallicity young SCs, and is very promising to explain the
association of many ultraluminous X-ray sources with low-metallicity and
actively star forming environments.
