Massive perturbers and the efficient merger of binary massive black holes

TL;DR

Massive perturbers like molecular clouds significantly accelerate the merger of binary massive black holes by enhancing stellar interactions, leading to rapid coalescence and increased gravitational wave event rates.

Contribution

This study demonstrates that massive perturbers dramatically speed up binary MBH mergers, a process previously limited by slower stellar relaxation mechanisms.

Findings

Massive perturbers accelerate stellar relaxation by orders of magnitude.

Binary MBHs in gas-rich nuclei coalesce within a Hubble time.

Lower-mass binary MBHs require only a small gas fraction to merge within cosmic timescales.

Abstract

We show that dynamical relaxation in the aftermath of a galactic merger and the ensuing formation and decay of a binary massive black hole (MBH), are dominated by massive perturbers (MPs) such as giant molecular clouds or clusters. MPs accelerate relaxation by orders of magnitude relative to 2-body stellar relaxation alone, and efficiently scatter stars into the binary MBH's orbit. The 3-body star-binary MBH interactions shrink the binary MBH to the point where energy losses from the emission of gravitational waves (GW) lead to rapid coalescence. We model this process based on observed and simulated MP distributions and take into account the decreased efficiency of the star-binary MBH interaction due to acceleration in the galactic potential. We show that mergers of gas-rich galactic nuclei lead to binary MBH coalescence well within the Hubble time. Moreover, lower-mass binary MBHs (<10^8 Msun) require only a few percent of the typical gas mass in a post-merger nucleus to coalesce in a Hubble time. The fate of a binary MBH in a gas poor galactic merger is less certain, although massive stellar structures (e.g. clusters, stellar rings) could likewise lead to efficient coalescence. These coalescence events are observable by their strong GW emission. MPs thus increase the cosmic rate of such GW events, lead to a higher mass deficit in the merged galactic core and suppress the formation of triple MBH systems and the resulting ejection of MBHs into intergalactic space.