Orbital Migration of Interacting Stellar Mass Black Holes in Disks around Supermassive Black Holes

TL;DR

This paper models the migration and formation of stellar-mass black hole binaries in active galactic nucleus disks, showing rapid binary formation at migration traps and potential contribution to observed gravitational wave merger rates.

Contribution

It introduces an augmented N-body simulation including gas torques, turbulence, and damping effects to study black hole migration and binary formation in AGN disks.

Findings

Black hole binaries form rapidly at migration traps.

Merging black hole binaries can form on short timescales.

AGN disks may significantly contribute to LIGO's observed merger rates.

Abstract

The merger rate of stellar-mass black hole binaries (sBHBs) inferred by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests the need for an efficient source of sBHB formation. Active galactic nucleus (AGN) disks are a promising location for the formation of these sBHBs, as well as binaries of other compact objects, because of powerful torques exerted by the gas disk. These gas torques cause orbiting compact objects to migrate towards regions in the disk where inward and outward torques cancel, known as migration traps. We simulate the migration of stellar mass black holes in an example of a model AGN disk, using an augmented N-body code that includes analytic approximations to migration torques, stochastic gravitational forces exerted by turbulent density fluctuations in the disk, and inclination and eccentricity dampening produced by passages through the gas disk, in addition to the standard gravitational forces between objects. We find that sBHBs form rapidly in our model disk as stellar-mass black holes migrate towards the migration trap. These sBHBs are likely to subsequently merge on short time-scales. The process continues, leading to the build-up of a population of over-massive stellar-mass black holes. The formation of sBHBs in AGN disks could contribute significantly to the sBHB merger rate inferred by LIGO.