Galaxy Rotation and Rapid Supermassive Black Hole Binary Coalescence

TL;DR

This study uses N-body simulations to show that galaxy rotation significantly accelerates supermassive black hole binary coalescence, especially in corotating and counterrotating flattened galaxy models, impacting gravitational wave detection.

Contribution

It demonstrates that galaxy rotation enhances SMBH binary hardening rates and accelerates coalescence, providing new insights into galaxy merger dynamics beyond static models.

Findings

Rotation increases SMBH binary hardening rates.

Corotating binaries settle into resonance orbits, speeding coalescence.

Counterrotating binaries coalesce rapidly on radial orbits.

Abstract

During a galaxy merger, the supermassive black hole (SMBH) in each galaxy is thought to sink to the center of the potential and form a supermassive black hole binary; this binary can eject stars via 3-body scattering, bringing the SMBHs ever closer. In a static spherical galaxy model, the binary stalls at a separation of about a parsec after ejecting all the stars in its loss cone -- this is the well-known final parsec problem. However it has been shown that SMBH binaries in non-spherical galactic nuclei harden at a nearly constant rate until reaching the gravitational wave regime. Here we use a suite of direct N-body simulations to follow SMBH binary evolution in both corotating and counterrotating flattened galaxy models. For N larger than 500K, we find that the evolution of the SMBH binary is convergent, and is independent of the particle number. Rotation in general increases the hardening rate of SMBH binaries even more effectively than galaxy geometry alone. SMBH binary hardening rates are similar for co- and counterrotating galaxies. In the corotating case, the center of mass of SMBH binary settles into an orbit that is in a corotation resonance with the background rotating model, and the coalescence time is roughly few hundred Myr faster than a non-rotating flattened model. We find that counterrotation drives SMBHs to coalesce on a nearly radial orbit promptly after forming a hard binary. We discuss the implications for gravitational wave astronomy, hypervelocity star production, and the effect on the structure of the host galaxy.