Star Cluster Disruption by a Supermassive Black Hole Binary

TL;DR

This paper investigates how infalling star clusters can accelerate the shrinking of supermassive black hole binaries, potentially influencing their evolution towards gravitational wave emission.

Contribution

It demonstrates through N-body simulations that radial star cluster infall can significantly hasten BHB hardening, unlike non-zero angular momentum infall.

Findings

Radial star cluster infall reduces BHB separation by ~10% in less than 10 Myr.

Non-zero angular momentum infall does not significantly enhance BHB hardening.

Star cluster infall can shorten the BHB evolution timescale toward gravitational wave emission.

Abstract

Supermassive black hole binaries (BHBs) are expected to be one of the most powerful sources of low-frequency gravitational waves (GWs) for future space-borne detectors. Prior to the GW emission stage, BHBs evolving in gas-poor nuclei shrink primarily through the slingshot ejection of stars approaching the BHB from sufficiently close distances. Here we address the possibility that the BHB shrinking rate is enhanced through the infall of a star cluster (SC) onto the BHB. We present the results of direct summation N-body simulations exploring different orbits for the SC infall, and we show that SCs reaching the BHB on non-zero angular momentum orbits (with eccentricity 0.75) fail to enhance the BHB hardening, while SCs approaching the BHB on radial orbits reduce the BHB separation by ~10% in less than 10 Myr, effectively shortening the BHB path towards GWs.