Low metallicity and ultra-luminous X-ray sources in the Cartwheel galaxy

Low-metallicity (Z <~ 0.05Z_solar) massive (>~40M_solar) stars might end their life by directly collapsing into massive black holes (BHs, 30 <~ m_BH/M_solar <~ 80). More than ~10⁵ massive BHs might have been generated via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ~10⁷yr. We show that such BHs might power most of the ultra-luminous X-ray sources (ULXs) observed in the Cartwheel. We also consider a sample of ULX-rich galaxies and we find a possible anticorrelation between the number of ULXs per galaxy and the metallicity in these galaxies. However, the data are not sufficient to draw any robust conclusions about this anticorrelation, and further studies are required.