Evolution of the Black Hole Mass Function in Star Clusters from Multiple Mergers

TL;DR

This paper models how black hole mergers in star clusters influence the black hole mass distribution, revealing potential observable features and the formation of intermediate mass black hole seeds.

Contribution

It introduces an evolving coagulation equation model to study the impact of mergers on black hole mass gaps and the formation of intermediate mass black holes.

Findings

Black hole mergers can fill the upper mass gap in the black hole mass function.

Lower mass gaps may produce observable features at larger masses.

Dynamical ejections influence black hole mass distribution and seed formation.

Abstract

We investigate the effects of black hole mergers in star clusters on the black hole mass function. As black holes are not produced in pair-instability supernovae, it is suggested that there is a dearth of high mass stellar black holes. This dearth generates a gap in the upper end of the black hole mass function. Meanwhile, parameter fitting of X-ray binaries suggests the existence of a gap in the mass function under 5 solar masses. We show, through evolving a coagulation equation, that black hole mergers can appreciably fill the upper mass gap, and that the lower mass gap generates potentially observable features at larger mass scales. We also explore the importance of ejections in such systems and whether dynamical clusters can be formation sites of intermediate mass black hole seeds.