Low metallicity and ultra-luminous X-ray sources in the Cartwheel galaxy
Abstract
Low-metallicity (Z <~ 0.05 Zsun) massive (>~40 Msun) stars might end their life by directly collapsing into massive black holes (BHs, 30 <~ mBH/Msun <~ 80). More than ~105 massive BHs might have been generated via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ~107 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.
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