Unravelling the progenitors of merging black hole binaries

TL;DR

This paper uses an advanced population-synthesis model to explore the origins of merging black hole binaries, highlighting the importance of low metallicity environments for their formation and merger rates.

Contribution

The study introduces MOBSE, an improved binary population-synthesis code, to analyze black hole binary formation, emphasizing the role of stellar metallicity.

Findings

Massive black hole binaries up to 120 solar masses form at low metallicity.

Only systems with total mass up to 80 solar masses merge within a Hubble time.

Merging black hole binaries are more efficiently formed from metal-poor stars.

Abstract

The recent detection of gravitational waves has proven the existence of massive stellar black hole binaries (BBHs), but the formation channels of BBHs are still an open question. Here, we investigate the demography of BBHs by using our new population-synthesis code MOBSE. MOBSE is an updated version of the widely used binary population-synthesis code BSE (Hurley et al. 2000, Hurley et al. 2002) and includes the key ingredients to determine the fate of massive stars: up-to-date stellar wind prescriptions and supernova models. With MOBSE, we form BBHs with total mass up to $\sim{}120$ M$_\odot$ at low metallicity, but only systems with total mass up to $\sim{}80$ M$_\odot$ merge in less than a Hubble time. Our results show that only massive metal-poor stars ($Z\lesssim 0.002$) can be the progenitors of gravitational wave events like GW150914. Moreover, we predict that merging BBHs form much more efficiently from metal-poor than from metal-rich stars.