The origin channels of hierarchical binary black hole mergers in the LIGO-Virgo-KAGRA O1, O2, and O3 runs

TL;DR

This study uses Bayesian analysis to determine the origins of hierarchical black hole mergers in LVK data, finding nuclear star clusters likely dominate and highlighting challenges in identifying formation environments from merger parameters.

Contribution

It introduces a hierarchical Bayesian framework to infer the dominant formation channels of hierarchical black hole mergers in GW data, emphasizing the role of NSCs and the difficulty of environment inference.

Findings

NSCs likely dominate hierarchical merger rate

Up to 50% of hierarchical mergers may originate from AGN disks

Hierarchical mergers account for at least 10% of LVK GW events

Abstract

We infer the origin channels of hierarchical mergers observed in the LIGO-Virgo-KAGRA (LVK) O1, O2, and O3 runs using a hierarchical Bayesian analysis under a parametric population model. By assuming the active galactic nucleus (AGN) disk and nuclear star cluster (NSC) channels, we find that NSCs likely dominate the hierarchical merger rate in the Universe, corresponding to a fraction of $f_{\rm NSC}=0.87_{-0.29}^{+0.10}$ at 90\% credible intervals in our fiducial model; AGN disks may contribute up to nearly half of hierarchical mergers detectable with LVK, specifically $f_{\rm det,AGN}=0.34_{-0.26}^{+0.38}$. We investigate the impact of the escape speed, along with other population parameters on the branching fraction, suggesting that the mass, mass ratio, and spin of the sources play significant roles in population analysis. We show that hierarchical mergers constitute at least $\sim$$10\%$ of the gravitational wave events detected by LVK during the O1-O3 runs. Furthermore, we demonstrate that it is challenging to effectively infer detailed information about the host environment based solely on the distribution of black hole merger parameters if multiple formation channels are considered.