Infalling clouds on to supermassive black hole binaries - II. Binary evolution and the final parsec problem

TL;DR

This study uses numerical simulations to demonstrate that interactions with infalling molecular clouds can effectively drive the orbital evolution of supermassive black hole binaries, potentially resolving the final parsec problem within a few hundred million years.

Contribution

The paper introduces a new model showing how gas cloud interactions can accelerate binary evolution, overcoming the final parsec problem.

Findings

Binary evolution is dominated by angular momentum exchange through gas capture and accretion.

The binary can reach the gravitational wave regime within a few hundred million years.

Cloud interactions can overcome the final parsec problem regardless of stellar environment.

Abstract

The formation of massive black hole binaries (MBHBs) is an unavoidable outcome of galaxy evolution via successive mergers. However, the mechanism that drives their orbital evolution from parsec separations down to the gravitational wave (GW) dominated regime is poorly understood, and their final fate is still unclear. If such binaries are embedded in gas-rich and turbulent environments, as observed in remnants of galaxy mergers, the interaction with gas clumps (such as molecular clouds) may efficiently drive their orbital evolution. Using numerical simulations, we test this hypothesis by studying the dynamical evolution of an equal-mass, circular MBHB accreting infalling molecular clouds. We investigate different orbital configurations, modelling a total of 13 systems to explore different possible impact parameters and relative inclinations of the cloud-binary encounter. We focus our study on the prompt, transient phase during the first few orbits when the dynamical evolution of the binary is fastest, finding that this evolution is dominated by the exchange of angular momentum through gas capture by the individual black holes and accretion. Building on these results, we construct a simple model for evolving a MBHB interacting with a sequence of clouds, which are randomly drawn from reasonable populations with different levels of anisotropy in their angular momenta distributions. We show that the binary efficiently evolves down to the GW emission regime within a few hundred million years, overcoming the 'final parsec' problem regardless of the stellar distribution.