Hot Circumsingle Disks Drive Binary Black Hole Mergers in Active Galactic Nucleus Disks

TL;DR

This paper investigates how thermodynamic feedback from embedded binary black holes in active galactic nucleus disks influences their orbital evolution, showing that increased local temperature can promote mergers, thus supporting AGN disks as sites for black hole mergers.

Contribution

It introduces a simple thermodynamic model demonstrating that hotter minidisks can reverse binary black hole orbital expansion to contraction, impacting merger rates in AGN disks.

Findings

Hotter minidisks cause binary contraction instead of expansion.

Thermodynamics significantly influence binary black hole orbital evolution.

AGN disks may be efficient environments for black hole mergers.

Abstract

Binary black hole (BBH) mergers, particularly those with component masses in the pair-instability gap, may be produced by hierarchical mergers in the disks surrounding Active Galactic Nuclei (AGN). While the interaction of an embedded BBH with an AGN disk is typically assumed to facilitate a merger, recent high-resolution hydrodynamical simulations challenge this assumption. However, these simulations often have simplified treatments for the gas thermodynamics. In this work, we model the possible consequence of various feedback from an embedded BBH with a simple model that maintains an enhanced temperature profile around each binary component. We show that when the minidisks around each BH become hotter than the background by a factor of three, the BBH orbital evolution switches from expansion to contraction. By analyzing the gravitational torque profile, we find that this change in direction is driven by a weakening of the minidisk spirals and their positive torque on the binary. Our results highlight the important role of thermodynamics around BBHs and its effect on their orbital evolution, suggesting that AGN disks could be efficient factories for BBH mergers.