Linking the fate of massive black hole binaries to the active galactic nuclei luminosity function

TL;DR

This study links the evolution of massive black hole binaries to the active galactic nuclei luminosity function, suggesting gas-driven decay efficiently leads to coalescence, impacting gravitational wave detection prospects.

Contribution

It introduces a novel approach using observational AGN data to constrain gas-driven binary decay efficiency and coalescence timelines.

Findings

Gas accretion can lead binaries to coalescence within the current age of the universe.

Downsizing trend implies light black holes can coalesce at lower redshifts.

Implications for future gravitational wave detection rates are significant.

Abstract

Massive black hole binaries are naturally predicted in the context of the hierarchical model of structure formation. The binaries that manage to lose most of their angular momentum can coalesce to form a single remnant. In the last stages of this process, the holes undergo an extremely loud phase of gravitational wave emission, possibly detectable by current and future probes. The theoretical effort towards obtaining a coherent physical picture of the binary path down to coalescence is still underway. In this paper, for the first time, we take advantage of observational studies of active galactic nuclei evolution to constrain the efficiency of gas-driven binary decay. Under conservative assumptions we find that gas accretion toward the nuclear black holes can efficiently lead binaries of any mass forming at high redshift (> 2) to coalescence within the current time. The observed "downsizing" trend of the accreting black hole luminosity function further implies that the gas inflow is sufficient to drive light black holes down to coalescence, even if they bind in binaries at lower redshifts, down to z~0.5 for binaries of ~10 million solar masses, and z~0.2 for binaries of ~1 million solar masses. This has strong implications for the detection rates of coalescing black hole binaries of future space-based gravitational wave experiments.