3D Adiabatic Simulations of Binary Black Hole Formation in AGN

TL;DR

This study uses 3D hydrodynamical simulations to explore how binary black holes form within AGN discs, revealing the influence of gas dynamics, outflows, and environmental factors on binary formation likelihood.

Contribution

It provides the first 3D non-isothermal hydrodynamical simulations of gas-assisted binary black hole formation in AGN, including models for predicting dissipation and formation likelihood.

Findings

Gas within the Hill sphere resembles a star with convective envelope.

Coriolis force induces winds that counter-rotate within the Hill sphere.

Binary formation is more likely in environments with larger gas mass in the Hill sphere.

Abstract

We investigate close encounters between initially unbound black holes (BHs) in the gaseous discs of active galactic nuclei (AGN), performing the first 3D non-isothermal hydrodynamical simulations of gas-assisted binary BH formation. We discuss a suite of 135 simulations, considering 9 AGN disc environments and 15 BH impact parameters. We find that the gas distribution within the Hill sphere about an isolated embedded BH is akin to a spherically symmetric star with a low-mass convective envelope and a BH core, with large convective currents driving strong outflows away from the midplane. We find that Coriolis force acting on the outflow results in winds, analogous to cyclones, that counter-rotate with respect to the midplane flow within the Hill sphere. We confirm the existence of strong thermal blasts due to minidisc collisions during BH close encounters, as predicted in our previous 2D studies. We document binary formation across a wide range of environments, finding formation likelihood is increased when the gas mass in the Hill sphere is large, allowing for easier binary formation in the outer AGN disc. We provide a comprehensive overview of the SMBH's role in binary formation, investigating how binary formation in intermediate density environments is biased towards certain binary orientations. We offer two models for predicting dissipation by gas during close encounters, as a function of the ambient Hill mass alone, or with the periapsis depth. We use these models to motivate a prescription for binary formation likelihood that can be readily applied to Monte-Carlo simulations of AGN evolution.