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

摘要

We investigate a purely stellar dynamical solution to the Final ParsecProblem. Galactic nuclei resulting from major mergers are not spherical, butshow some degree of triaxiality. With $N$-body simulations, we show thatmassive black hole binaries (MBHB) hosted by them will continuously interactwith stars on centrophilic orbits and will thus inspiral---in much less than aHubble time---down to separations at which gravitational wave (GW) emission isstrong enough to drive them to coalescence. Such coalescences will be importantsources of GWs for future space-borne detectors such as the {\it LaserInterferometer Space Antenna} (LISA). Based on our results, we expect that LISAwill see between $\sim 10$ to $\sim {\rm few} \times 10^2$ such events everyyear, depending on the particular MBH seed model as obtained in recent studiesof merger trees of galaxy and MBH co-evolution. Orbital eccentricities in theLISA band will be clearly distinguishable from zero with $e \gtrsim0.001-0.01$.