Fast coalescence of massive black hole binaries from mergers of galactic nuclei: implications for low-frequency gravitational-wave astrophysics

TL;DR

This study demonstrates that non-spherical galactic nuclei facilitate rapid inspiral and coalescence of massive black hole binaries through stellar interactions, significantly impacting gravitational wave detection prospects with LISA.

Contribution

It provides a stellar dynamical solution to the Final Parsec Problem, showing how galactic triaxiality enables efficient black hole binary coalescence within a Hubble time.

Findings

Black hole binaries inspiral rapidly in triaxial nuclei.

LISA will detect approximately 10 to hundreds of such events annually.

Orbital eccentricities in the LISA band are expected to be above 0.001-0.01.

Abstract

We investigate a purely stellar dynamical solution to the Final Parsec Problem. Galactic nuclei resulting from major mergers are not spherical, but show some degree of triaxiality. With $N$-body simulations, we show that massive black hole binaries (MBHB) hosted by them will continuously interact with stars on centrophilic orbits and will thus inspiral---in much less than a Hubble time---down to separations at which gravitational wave (GW) emission is strong enough to drive them to coalescence. Such coalescences will be important sources of GWs for future space-borne detectors such as the {\it Laser Interferometer Space Antenna} (LISA). Based on our results, we expect that LISA will see between $\sim 10$ to $\sim {\rm few} \times 10^2$ such events every year, depending on the particular MBH seed model as obtained in recent studies of merger trees of galaxy and MBH co-evolution. Orbital eccentricities in the LISA band will be clearly distinguishable from zero with $e \gtrsim 0.001-0.01$.