Remnants of massive metal-poor stars: viable engines for ultra-luminous
  X-ray sources
  Massive metal-poor stars might end their life by directly collapsing into
massive (~25-80 Msun) black holes (BHs). We derive the number of massive BHs
(N_BH) that are expected to form per galaxy via this mechanism. We select a
sample of 66 galaxies with X-ray coverage, measurements of the star formation
rate (SFR) and of the metallicity. We find that N_BH correlates with the number
of observed ultra-luminous X-ray sources (ULXs) per galaxy (N_ULX) in this
sample. We discuss the dependence of N_ULX and of N_BH on the SFR and on the
metallicity.
