Hydrodynamical Simulations of Black-Hole Binary Formation in AGN Disks

TL;DR

This paper uses hydrodynamical simulations to explore how black hole binaries can form in AGN disks through close encounters, emphasizing the role of gas dynamics and providing predictive formulas for binary formation.

Contribution

It introduces a new simulation-based analysis of black hole binary formation in AGN disks and offers a practical fitting formula for predicting binary formation outcomes.

Findings

Bound black hole binaries can form via circum-single disk collisions in dense AGN disks.

Gas post-collision drag can tighten black hole pairs into binaries with high eccentricity.

A fitting formula accurately predicts binary formation likelihood based on gas mass and encounter energy.

Abstract

We study the close encounters between two single black holes (BHs) embedded in an AGN disk using a series of global 2D hydrodynamics simulations. We find that when the disk density is sufficiently high, bound BH binaries can be formed by the collision of their circum-single disks. Our analysis demonstrates that, after a BH pair passes the pericenter of their relative trajectory, a gas post-collision drag may slow down the BHs, possibly forcing the two BHs to stay tightly bound. A binary formed by a close encounter can have a compact semi-major axis, large eccentricity, and retrograde orbital angular momentum. We provide a fitting formula that can accurately predict whether a close encounter can form a binary based on the gas mass and the incoming energy of the encounter. This fitting formula can be easily implemented in other long-term simulations that study the dynamical evolution of BHs in AGN disks.