On the Origin of GW190521-like events from repeated black hole mergers in star clusters

TL;DR

This paper investigates how repeated black hole mergers in dense star clusters can produce very massive black holes like GW190521, emphasizing the roles of cluster properties and merger dynamics.

Contribution

It demonstrates that black holes of around 150 solar masses can form through repeated mergers in clusters with high escape velocities, regardless of metallicity.

Findings

Massive black holes can form in clusters with escape speeds >200 km/s.

Detection probability increases with secondary mass and decreases with primary mass and redshift.

Repeated mergers are a viable pathway for forming intermediate-mass black holes.

Abstract

LIGO and Virgo have reported the detection of GW190521, from the merger of a binary black hole (BBH) with a total mass around $150$ M$_\odot$. While current stellar models limit the mass of any black hole (BH) remnant to about $40 - 50$ M$_\odot$, more massive BHs can be produced dynamically through repeated mergers in the core of a dense star cluster. The process is limited by the recoil kick (due to anisotropic emission of gravitational radiation) imparted to merger remnants, which can escape the parent cluster, thereby terminating growth. We study the role of the host cluster metallicity and escape speed in the buildup of massive BHs through repeated mergers. Almost independent of host metallicity, we find that a BBH of about $150$ M$_\odot$ could be formed dynamically in any star cluster with escape speed $\gtrsim 200$ km s$^{-1}$, as found in galactic nuclear star clusters as well as the most massive globular clusters and super star clusters. Using an inspiral-only waveform, we compute the detection probability for different primary masses ($\ge 60$ M$_\odot$) as a function of secondary mass and find that the detection probability increases with secondary mass and decreases for larger primary mass and redshift. Future additional detections of massive BBH mergers will be of fundamental importance for understanding the growth of massive BHs through dynamics and the formation of intermediate-mass BHs.