Comparing hierarchical black hole mergers in star clusters and active galactic nuclei by Guo-Peng Li et al. on Tuesday 22 November

Star clusters (SCs) and active galactic nuclei (AGNs) are promising sites for

the occurrence of hierarchical black hole (BH) mergers. We use simple models to

compare hierarchical BH mergers in two of the dynamical formation channels. We

find that the primary mass distribution of hierarchical mergers in AGNs is

higher than that in SCs, with the peaks of $\sim$$13~M_{\odot}$ and

$\sim$$50~M_{\odot}$, respectively. The effective spin ($\chi_{\rm eff}$)

distribution of hierarchical mergers in SCs is symmetrical around zero as

expected and $\sim$50\% of the mergers have $|\chi_{\rm eff}|>0.2$. The

distribution of $\chi_{\rm eff}$ in AGNs is narrow and prefers positive values

with the peak of $\chi_{\rm eff}\ge0.3$ due to the assistance of AGN disks. BH

hierarchical growth efficiency, with at least $\sim$30\% of mergers being

hierarchies in AGNs, is much higher than the efficiency in SCs. We argue that

the majority of the hierarchical merger candidates detected by LIGO-Virgo may

originate from the AGN channel.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2211.11150v1