Binary black hole mergers from Population III star clusters

TL;DR

This study explores how cluster dynamics influence the mass distribution of Population III binary black hole mergers, revealing a significant fraction with masses above the pair-instability gap and estimating high merger rates at early cosmic times.

Contribution

It provides the first detailed analysis of the impact of cluster dynamics on Pop. III BBH mass functions and merger rates, highlighting the importance of dynamical processes in high-redshift black hole formation.

Findings

A significant fraction (5-100%) of Pop. III BBH mergers have primary masses above the pair-instability gap.

Dynamical formation leads to higher merger rates (~200 Gpc^{-3} yr^{-1}) at z~15 compared to isolated binaries.

A zone of avoidance exists in the primary mass - mass ratio plane, with sharp boundaries.

Abstract

Binary black holes (BBHs) born from the evolution of Population III (Pop. III) stars are one of the main high-redshift targets for next-generation ground-based gravitational-wave (GW) detectors. Their predicted initial mass function and lack of metals make them the ideal progenitors of black holes above the upper edge of the pair-instability mass gap, i.e. with a mass higher than $\approx{}134$ (241) M$_\odot$ for stars that become (do not become) chemically homogeneous during their evolution. Here, we investigate the effects of cluster dynamics on the mass function of BBHs born from Pop. III stars, by considering the main uncertainties on Pop. III star mass function, orbital properties of binary systems, star cluster's mass and disruption time. In our dynamical models, at least $\sim$5% and up to 100% BBH mergers in Pop. III star clusters have primary mass $m_1$ above the upper edge of the pair-instability mass gap. In contrast, only $\lesssim {} 3$% isolated BBH mergers have primary mass above the gap, unless their progenitors evolved as chemically homogeneous stars. The lack of systems with primary and/or secondary mass inside the gap defines a zone of avoidance with sharp boundaries in the primary mass - mass ratio plane. Finally, we estimate the merger rate density of BBHs and, in the most optimistic case, we find a maximum of $\mathcal{R}\approx200\,{\rm Gpc^{-3}\,yr^{-1}}$ at $z\sim15$ for BBHs formed via dynamical capture. For comparison, the merger rate density of isolated Pop. III BBHs is $\mathcal{R}\leq{}10\,{\rm Gpc^{-3}\,yr^{-1}}$, for the same model of Pop. III star formation history.