Remnants of massive metal-poor stars: viable engines for ultra-luminous X-ray sources

TL;DR

This paper investigates how massive, metal-poor stars can directly collapse into black holes and potentially explain the origins of ultra-luminous X-ray sources in galaxies.

Contribution

It quantifies the expected number of massive black holes formed from metal-poor stars and correlates this with observed ULX counts across a galaxy sample.

Findings

N_BH correlates with N_ULX in the galaxy sample.

The number of black holes depends on star formation rate and metallicity.

Metal-poor environments favor the formation of ULXs from massive black holes.

Abstract

Massive metal-poor stars might end their life by directly collapsing into massive (~25-80 Msun) 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.