Black hole merger rates in AGN: contribution from gas-captured binaries

ABSTRACT Merging black hole (BH) binaries in active galactic nucleus (AGN) discs formed through two-body scatterings via the ‘gas-capture’ process may explain a significant fraction of BH mergers in AGN and a non-negligible contribution to the observed rate from LIGO-VIRGO-KAGRA. We perform Monte Carlo simulations of binary BH formation, evolution, and mergers across the observed AGN mass function using a novel physically motivated treatment for the gas-capture process derived from hydrodynamical simulations of BH–BH encounters in AGN. Our models suggest that gas-captured binaries could result in merger rates of $0.73-7.1$ Gpc$^{-3}$ yr$^{-1}$. Mergers from AGN are dominated by AGN with supermassive BH masses of $\sim 10^{7}\, \mathrm{M}_{\odot }$, with 90 per cent of mergers occurring in the range $\sim 10^{6}\, \mathrm{M}_{\odot }-10^{8}\, \mathrm{M}_{\odot }$. The merging mass distribution is flatter than the initial BH mass power law by a factor $\Delta \xi =1.1-1.2$, as larger BHs align with the disc and form binaries more efficiently. Similarly, the merging mass ratio distribution is flatter therefore the AGN channel could explain high mass and unequal mass ratio detections such as GW190521 and GW190814. Using a simpler dynamical friction treatment for the binary formation process, the results are similar, where the primary bottleneck is the alignment time with the disc. The most influential parameters are the anticipated number of BHs and their mass function. Given the many uncertainties that remain in the AGN channel, we expect the true uncertainty extends beyond our predicted rates. None the less, we conclude that AGN remain an important channel for consideration, particularly for gravitational wave detections involving one or two high mass BHs.