Accretion of clumpy cold gas onto massive black hole binaries: a possible fast route to binary coalescence

TL;DR

This study uses hydrodynamical simulations to show that infalling cold gas clumps in galactic nuclei can significantly accelerate the coalescence of massive black hole binaries by enhancing angular momentum exchange.

Contribution

It introduces a new model of sequential cold gas cloud accretion, demonstrating its role in speeding up binary black hole coalescence compared to previous isolated cloud interactions.

Findings

Accretion increases with multiple infalling clouds.

Binary evolution is faster with sequential accretion.

Gas inflow promotes efficient angular momentum exchange.

Abstract

In currently favoured hierarchical cosmologies, the formation of massive black hole binaries (MBHBs) following galaxy mergers is unavoidable. Still, due the complex physics governing the (hydro)dynamics of the post-merger dense environment of stars and gas in galactic nuclei, the final fate of those MBHBs is still unclear. In gas-rich environments, it is plausible that turbulence and gravitational instabilities feed gas to the nucleus in the form of a series of cold incoherent clumps, thus providing a way to exchange energy and angular momentum between the MBHB and its surroundings. Within this context, we present a suite of smoothed-particle-hydrodynamical models to study the evolution of a sequence of near-radial turbulent gas clouds as they infall towards equal-mass, circular MBHBs. We focus on the dynamical response of the binary orbit to different levels of anisotropy of the incoherent accretion events. Compared to a model extrapolated from a set of individual cloud-MBHB interactions, we find that accretion increases considerably and the binary evolution is faster. This occurs because the continuous infall of clouds drags inwards circumbinary gas left behind by previous accretion events, thus promoting a more effective exchange of angular momentum between the MBHB and the gas. These results suggest that sub-parsec MBHBs efficiently evolve towards coalescence during the interaction with a sequence of individual gas pockets.