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

Low-metallicity (Z < ∼ 0.05Z⊙) massive ( > ∼ 40M⊙) stars might end their life by directly collapsing into massive black holes (BHs, 30 < ∼ mBH/M⊙ < ∼ 80). More than ∼ 10 massive BHs might have been generated via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ∼ 10 yr. 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 anti-correlation 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 anti-correlation, and further studies are required.