Star cluster disruption by a massive black hole binary

TL;DR

This study uses N-body simulations to investigate how an infalling stellar cluster affects the orbital decay of a massive black hole binary, highlighting the importance of the cluster's orbit in the process.

Contribution

It demonstrates that stellar clusters on radial orbits can significantly shrink black hole binaries, especially when combined with a stellar cusp, revealing a potential pathway for binary evolution.

Findings

Radial infall of stellar clusters causes >10% shrinkage of BHB semi-major axis.

Including a stellar cusp enhances the BHB shrinking effect.

Non-zero angular momentum orbits result in negligible BHB shrinking.

Abstract

Massive black hole binaries (BHBs) are expected to form as the result of galaxy mergers; they shrink via dynamical friction and stellar scatterings, until gravitational waves (GWs) bring them to the final coalescence. It has been argued that BHBs may stall at a parsec scale and never enter the GW stage if stars are not continuously supplied to the BHB loss cone. Here we perform several N-body experiments to study the effect of an 80,000 solar masses stellar cluster (SC) infalling on a parsec-scale BHB. We explore different orbital elements for the SC and we perform runs both with and without accounting for the influence of a rigid stellar cusp (modelled as a rigid Dehnen potential). We find that the semi-major axis of the BHB shrinks by more than 10 per cent if the SC is on a nearly radial orbit; the shrinking is more efficient when a Dehnen potential is included and the orbital plane of the SC coincides with that of the BHB. In contrast, if the SC orbit has non-zero angular momentum, only a few stars enter the BHB loss cone and the resulting BHB shrinking is negligible. Our results indicate that SC disruption might significantly contribute to the shrinking of a parsec-scale BHB only if the SC approaches the BHB on a nearly radial orbit.