Impact of gas hardening on the population properties of hierarchical black hole mergers in AGN disks

TL;DR

This paper investigates how gas accretion and migration traps in AGN disks influence hierarchical black hole mergers, revealing that gas hardening significantly enhances merger efficiency and affects spin and mass correlations.

Contribution

It introduces a new semi-analytic model to study gas hardening effects on black hole merger hierarchies in AGNs, highlighting its role in forming intermediate-mass black holes.

Findings

Gas hardening greatly increases hierarchical merger efficiency.

Efficient gas hardening leads to spin alignment and specific mass-spin correlations.

Inefficient gas hardening truncates the merger chain after few generations.

Abstract

Hierarchical black hole (BH) mergers in active galactic nuclei (AGNs) are unique among formation channels of binary black holes (BBHs) because they are likely associated with electromagnetic counterparts and can efficiently lead to the mass growth of BHs. Here, we explore the impact of gas accretion and migration traps on the evolution of BBHs in AGNs. We have developed a new fast semi-analytic model, which allows us to explore the parameter space while capturing the main physical processes involved. We find that effective exchange of energy and angular momentum between the BBH and the surrounding gas (hereafter, gas hardening) during inspiral greatly enhances the efficiency of hierarchical mergers, leading to the formation of intermediate-mass BHs (up to 10.000 solar masses) and triggering spin alignment. Moreover, our models with efficient gas hardening show both an anti-correlation between BBH mass ratio and effective spin, and a correlation between primary BH mass and effective spin. In contrast, if gas hardening is inefficient, the hierarchical merger chain is already truncated after the first two or three generations. We compare the BBH population in AGNs with other dynamical channels as well as isolated binary evolution.