The missing link between ultraluminous X-ray sources and metallicity
  The nature of ultraluminous X-ray sources (ULXs) is still debated. Recent
studies show that metal-poor massive stars can collapse into massive stellar
black holes (MSBHs), that is black holes with mass > 25 Msun. Such MSBHs are
sufficiently massive to explain most ULXs without requiring substantial
violations of the Eddington limit. The recent finding of an anti-correlation
between metallicity of the environment and number of ULXs per galaxy supports
this hypothesis. We present the results of recent N-body simulations, including
metallicity dependent stellar evolution, and we discuss the main pathways to
produce X-ray binaries powered by MSBHs.
