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 (approximate to 25-80M(circle dot)) 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. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim