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

TL;DR

This paper suggests that low-metallicity massive stars in the Cartwheel galaxy could produce numerous massive black holes that power most of its ultra-luminous X-ray sources, and explores a potential anti-correlation between ULXs and metallicity.

Contribution

It introduces the idea that low-metallicity environments generate many massive black holes that explain ULXs in the Cartwheel galaxy, a novel connection between metallicity and ULX populations.

Findings

Potentially over 10^5 massive black holes formed in the Cartwheel

Most ULXs in the Cartwheel may be powered by these black holes

Possible anti-correlation between ULX number and galaxy metallicity

Abstract

Low-metallicity (Z <~ 0.05 Zsun) massive (>~40 Msun) stars might end their life by directly collapsing into massive black holes (BHs, 30 <~ m_BH/Msun <~ 80). More than ~10^5 massive BHs might have been generated via this mechanism in the metal-poor ring galaxy Cartwheel, during the last ~10^7 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.