Cosmic merger rates and population evolution of binary black hole mergers in AGN disks

Abstract

Active galactic nuclei (AGN) disks are promising sites for hierarchical binary black hole (BBH) mergers, which next-generation gravitational-wave detectors will probe out to high redshift. Predicting their contribution requires linking BBH evolution in AGN disks to the poorly constrained high-redshift evolution of active supermassive black holes (SMBHs). We introduce ARISE (AGN Rates and Intrinsic Stellar-BH Evolution), a framework that embeds semi-analytical models of BBH evolution in AGN disks into a cosmological description of the AGN number density, SMBH mass function, Eddington-ratio distribution, and nuclear metallicity. We consider a conservative AGN-abundance model anchored to observed luminosity functions, as well as Eddington-limited and super-Eddington scenarios based on the CAT cosmological model, compatible with JWST observations of high-redshift AGNs. The merger-rate density is set primarily by the disk viscosity parameter $α$: at $z = 0.2$, we find a rate of $12$--$37$ Gpc$^{-3}$ yr$^{-1}$ for $α= 0.01$ and $0.5$--$6$ Gpc$^{-3}$ yr$^{-1}$ for $α= 0.1$. The redshift evolution depends strongly on how SMBHs grow. The rate density peaks at $z \simeq 1.8$, $3.3$, and $3.9$ ($1.5$, $3.6$, and $3.9$) for $α=0.01$ ($0.1$) in the conservative, Eddington-limited, and super-Eddington models, respectively, and in the super-Eddington model it stays nearly flat out to $z \simeq 10$. For $α= 0.1$, AGN disks account for $\sim 2$--$40\%$ of the LVK-inferred merger rate, consistent with current upper limits from GW--AGN associations. For $α= 0.01$, they match or exceed it. The Einstein Telescope (ET) should detect $\sim 10^{5}$ ($\sim 10^{4}$) AGN-disk mergers per year for $α= 0.01$ ($0.1$). ET will test the AGN channel at redshifts where the AGN population models differ most strongly, providing independent constraints on the growth of early SMBHs.