Contracting and Expanding Binary Black Holes in 3D Low-Mass AGN Disks: The Importance of Separation

TL;DR

This study uses 3D high-resolution simulations to explore how binary black holes embedded in AGN disks evolve, revealing that their orbital separation can either contract or expand depending on their distance, impacting merger likelihood.

Contribution

First 3D simulations of embedded BBHs in AGN disks analyzing mass accretion and orbital evolution at various separations, highlighting the complexity of merger pathways.

Findings

BBHs with >10% Hill radius separation contract and accrete super-Eddington.

Smaller separations lead to BBH orbital expansion.

Implications for the difficulty of BBH mergers in AGN disks.

Abstract

LIGO/Virgo has detected several binary black hole (BBH) merger events that may have originated in the accretion disks of Active Galactic Nuclei (AGN). These events require individual black hole masses that fall within the pair instability supernova mass gap, and therefore these black holes may have been grown from hierarchical mergers. AGN disks are a prime environment for hierarchical mergers, and thus a potential location for the progenitors of BBH gravitational wave events. Understanding how a BBH embedded in an AGN disk interacts with the surrounding environment is thus crucial for determining if this interaction can lead to its merger. However, there are few high fidelity simulations of this process, and almost all are two-dimensional. We present the results from 3D, high-resolution, local shearing-box simulations of an embedded BBH interacting with an AGN disk. In these first simulations of their kind, we focus on determining the mass accretion rate and the orbital evolution rate at different BBH separations. We find that circular, equal-mass BBHs with separations greater than 10% of their Hill radius contract while accreting at a super-Eddington rate. At smaller separations, however, our 3D simulations find that BBHs expand their orbits. This result suggests that it may be difficult for an AGN disk to push a BBH to merger, but we discuss several mechanisms, including MHD turbulence and radiative and mechanical feedback, that could alleviate this difficulty.