The mass of stellar black holes (BHs) born from solar-metallicity
progenitors can hardly be higher than ~10 Msun. On the other hand,
massive metal-poor stars are expected to directly collapse into
massive (25-80 solar masses) stellar black holes (MSBHs). MSBHs might
play a role in a plethora of astrophysical processes, from the
dynamical evolution of star clusters, to the emission of gravitational
waves.
In this talk, I will focus on the possible connection between MSBHs
and ultra-luminous X-ray sources (ULXs). ULXs are point-like
off-centre sources with X-ray luminosity (assumed to be isotropic)
higher than the Eddington luminosity for a ~10 Msun BH. I will show
that the expected number of MSBHs per galaxy, predicted from the
theoretical model, correlates with the observed number of ULXs per
galaxy, in a sample of 66 late-type galaxies. Finally, I will discuss
the dependence of the model on metallicity, star formation rate and
dynamical evolution of the environment.