Hierarchical Black Hole Mergers in AGN Disks: Tracing Massive Black Hole Growth Across Cosmic Time

TL;DR

This paper models how active galactic nuclei (AGN) environments facilitate repeated black hole mergers, potentially explaining observed high-mass binary black holes and their spins, and offers a framework for future gravitational-wave studies.

Contribution

It introduces a semi-analytical population synthesis model capturing key physical processes in AGN disks for black hole mergers.

Findings

AGN disks can produce hierarchical black hole mergers.

Mergers near migration traps contribute to high-mass, high-spin black holes.

The model helps constrain the role of AGN in black hole growth.

Abstract

The dense and gaseous environments of active galactic nuclei (AGNs) can catalyze repeated mergers of stellar-mass black holes (BHs), potentially explaining the high-mass tail of binary black hole (BBH) mergers observed by LIGO-Virgo-KAGRA (LVK). We present a semi-analytical population synthesis framework that captures key physical processes in AGN disks, including gas capture, migration, binary pair-up, gas hardening, and dynamical binary-single interactions. Our simulations show that AGN disks can produce hierarchical mergers, especially near migration traps, and may contribute to the high-mass, high-spin BBH population. This work opens prospects for constraining both the AGN channel and supermassive black hole growth with future gravitational-wave detections.