Efficient Merger of Binary Supermassive Black Holes in Merging Galaxies

TL;DR

This study demonstrates that galaxy mergers with nonspherical, triaxial shapes facilitate rapid binary supermassive black hole mergers through stellar interactions, solving the final-parsec problem and enhancing gravitational wave detection prospects.

Contribution

The paper provides the first fully self-consistent merger simulations showing that galaxy shape asymmetries enable efficient SMBH binary hardening, addressing the final-parsec problem.

Findings

Binary SMBH hardening rates are significantly higher in nonspherical galaxies.

Hardening rates are largely independent of simulation particle number.

Binary eccentricities remain high throughout the process.

Abstract

In spherical galaxies, binary supermassive black holes (SMBHs) have difficulty reaching sub-parsec separations due to depletion of stars on orbits that intersect the massive binary - the final-parsec problem. Galaxies that form via major mergers are substantially nonspherical, and it has been argued that the centrophilic orbits in triaxial galaxies might provide stars to the massive binary at a high enough rate to avoid stalling. Here we test that idea by carrying out fully self-consistent merger simulations of galaxies containing central SMBHs. We find hardening rates of the massive binaries that are indeed much higher than in spherical models, and essentially independent of the number of particles used in the simulations. Binary eccentricities remain high throughout the simulations. Our results constitute a fully stellar-dynamical solution to the final-parsec problem and imply a potentially high rate of events for low-frequency gravitational wave detectors like LISA.