Rapid Formation of Supermassive Black Hole Binaries in Galaxy Mergers with Gas

TL;DR

This study uses hydrodynamical simulations to demonstrate that supermassive black hole binaries can form rapidly within a turbulent nuclear gas disk after galaxy mergers, driven primarily by gas dynamical processes.

Contribution

It provides the first detailed simulation showing SMBH binary formation at small scales during galaxy mergers with gas, highlighting the role of gas dynamics over stellar processes.

Findings

SMBH binaries form within less than a million years.

Gas dynamical drag is the main mechanism for binary formation.

A turbulent nuclear gas disk develops post-merger.

Abstract

Supermassive black holes (SMBHs) are a ubiquitous component of the nuclei of galaxies. It is normally assumed that, following the merger of two massive galaxies, a SMBH binary will form, shrink due to stellar or gas dynamical processes and ultimately coalesce by emitting a burst of gravitational waves. However, so far it has not been possible to show how two SMBHs bind during a galaxy merger with gas due to the difficulty of modeling a wide range of spatial scales. Here we report hydrodynamical simulations that track the formation of a SMBH binary down to scales of a few light years following the collision between two spiral galaxies. A massive, turbulent nuclear gaseous disk arises as a result of the galaxy merger. The black holes form an eccentric binary in the disk in less than a million years as a result of the gravitational drag from the gas rather than from the stars.