Black hole, neutron star and white dwarf merger rates in AGN disks

Abstract

Advanced LIGO \& Advanced Virgo are detecting a large number of binary stellar origin black hole (BH) mergers. A promising channel for accelerated BH merger lies in active galactic nucleus (AGN) disks of gas around super-masssive black holes. Here we investigate the relative number of compact object mergers in AGN disk models, including BH, neutron stars (NS) and white dwarfs, via Monte Carlo simulations. We find the number of all merger types in the bulk disk grows $\propto t^{1/3}$ which is driven by the Hill sphere of the more massive merger component. Median mass ratios of NS-BH mergers in AGN disks are $\tilde{q}=0.07\pm 0.06(0.14\pm 0.07)$ for mass functions (MF) $M^{-1(-2)}$. If a fraction $f_{AGN}$ of the observed rate of BH-BH mergers (${\cal}{R}_{BH-BH}$) come from AGN, we expect a rate of NS-BH (NS-NS) mergers from the AGN channel of ${\cal}{R}_{BH-NS} \sim f_{AGN}[10,300]\rm{Gpc}^{-3} \rm{yr}^{-1}$,(${\cal}{R}_{NS-NS} \leq f_{AGN}400\rm{Gpc}^{-3} \rm{yr}^{-1}$). Allowing for the ratio of NS-NS/BH-BH LIGO search volumes, from preliminary O3 results we can rule out the AGN channel as a dominant contribution to observed NS-NS mergers. The number of lower mass gap events in this channel is a strong function of the nuclear MF and mass segregation efficiency. Compact object merger ratios derived from LIGO can restrict models of MF, mass segregation and populations embedded in AGN disks. The expected number of EM counterparts to NS-BH mergers in AGN disks at $z<1$ is $\sim [30,900]{\rm{yr}}^{-1}(f_{AGN}/0.1)$. EM searches for flaring due to embedded events in large AGN surveys will complement LIGO constraints on AGN models and the embedded populations that must live in them.