Massive binary black holes in galactic nuclei and their path to coalescence

TL;DR

This paper reviews the processes and dynamics leading to the formation and coalescence of massive binary black holes in galactic nuclei, emphasizing their significance as gravitational wave sources and exploring various merger scenarios.

Contribution

It provides a comprehensive review of the current understanding of black hole binary formation and evolution in different galaxy merger environments.

Findings

Black hole binaries can form and coalesce in diverse merger scenarios.

Gas presence influences the binary evolution and coalescence timescales.

Progress has been made in understanding the dynamics from galaxy scales to nuclear regions.

Abstract

Massive binary black holes form at the centre of galaxies that experience a merger episode. They are expected to coalesce into a larger black hole, following the emission of gravitational waves. Coalescing massive binary black holes are among the loudest sources of gravitational waves in the Universe, and the detection of these events is at the frontier of contemporary astrophysics. Understanding the black hole binary formation path and dynamics in galaxy mergers is therefore mandatory. A key question poses: during a merger, will the black holes descend over time on closer orbits, form a Keplerian binary and coalesce shortly after? Here we review progress on the fate of black holes in both major and minor mergers of galaxies, either gas-free or gas-rich, in smooth and clumpy circum-nuclear discs after a galactic merger, and in circum-binary discs present on the smallest scales inside the relic nucleus.