Mass Function of Binary Massive Black Holes in Active Galactic Nuclei

TL;DR

This paper models the evolution and mass function of binary massive black holes in active galactic nuclei, predicting their prevalence and detectability with current X-ray observatories based on galaxy merger theories.

Contribution

It introduces a theoretical framework for the mass function and evolution timescale of binary black holes in AGNs, linking galaxy mergers to observable binary black hole populations.

Findings

Approximately 1.8% of nearby AGNs host close binary black holes with periods less than ten years.

About 20% of AGNs with specific black hole masses may contain detectable close binary black holes.

Binary black holes are predicted to merge within a Hubble time due to disk interactions.

Abstract

If the activity of active galactic nuclei (AGNs) is predominantly induced by major galaxy mergers, then a significant fraction of AGNs should harbor binary massive black holes in their centers. We study the mass function of binary massive black holes in nearby AGNs based on the observed AGN black-hole mass function and theory of evolution of binary massive black holes interacting with a massive circumbinary disk in the framework of coevolution of massive black holes and their host galaxies. The circumbinary disk is assumed to be steady, axisymmetric, geometrically thin, self-regulated, self-gravitating but non-fragmenting with a fraction of Eddington accretion rate, which is typically one tenth of Eddington value. The timescale of orbital decay is {then} estimated as ~10^8yr for equal mass black-hole, being independent of the black hole mass, semi-major axis, and viscosity parameter but dependent on the black-hole mass ratio, Eddington ratio, and mass-to-energy conversion efficiency. This makes it possible for any binary massive black holes to merge within a Hubble time by the binary-disk interaction. We find that (1.8+-0.6%) for the equal mass ratio and (1.6+-0.4%) for the one-tenth mass ratio of the total number of nearby AGNs have close binary massive black holes with orbital period less than ten years in their centers, detectable with on-going highly sensitive X-ray monitors such as Monitor of All-sky X-ray Image and/or Swift/Burst Alert Telescope. Assuming that all binary massive black holes have the equal mass ratio, about 20% of AGNs with black hole masses of 10^{6.5-7}M_sun has the close binaries and thus provides the best chance to detect them.