Binary black hole mergers from Population III star clusters

Abstract Binary black holes (BBHs) that are 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, that is, with a mass higher than $\sim{} 241~\mathrm{M}_\odot$. In Mestichelli et al. 2024, we investigated the effects of cluster dynamics on the mass function of BBHs that are born from Pop. III stars by considering the main uncertainties on the mass function of Pop. III stars, the orbital properties of the binary systems, the star cluster mass, and the disruption time. To do so, we used the semi-analytical code $\text{FASTCLUSTER}$ (Mapelli et al. 2021, Torniamenti et al. 2024). In our dynamical models, at least $\sim 5\%$ and up to $100\%$ BBH mergers in Pop. III star clusters have a primary mass $m_1$ above the upper edge of the pair-instability mass gap. In contrast, only $\lesssim {} 3\%$ isolated BBH mergers have a primary mass above the gap, unless their progenitors evolved as chemically homogeneous stars. The lack of systems with a primary and/or secondary mass inside the gap defines a zone of avoidance with sharp boundaries in the plane of the primary mass - mass ratio. Finally, we estimated the merger rate density of BBHs. In the most optimistic case, we found a maximum of $\mathcal{R}\sim200\,{\rm Gpc^{-3}\,yr^{-1}}$ at $z\sim15$ for BBHs that 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. Here we report the catalogs of BBH mergers and the merger rate densities as processed via $\text{COSMO}\mathcal{R}\text{ATE}$ (Santoliquido et al. 2020, 2021, 2023) obtained with the fiducial configuration from Mestichelli et al. 2024.