Fast Coalescence of Massive Black Hole Binaries From Mergers of Galactic Nuclei: Implications for Low-Frequency Gravitational-Wave Astrophysics

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 equal-mass massive black hole binaries (MBHBs) 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 Laser Interferometer Space Antenna (LISA). Based on our results for equal-mass mergers, and given that the hardening rate of unequal-mass binaries is similar, we expect that LISA will see between ∼10 and ∼ few × 10 2 such events every year, depending on the particular massive black hole (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 0.001-0.01.

The puzzling case of the radio-loud QSO 3C 186: a gravitational wave recoiling black hole in a young radio source?

“…Recent work using simulations also show that even in gas-poor environments SMBH binaries can merge under certain conditions, e.g. if they formed in major galaxy mergers where the final galaxy is non-spherical (Khan et al 2011;Preto et al 2011;Khan et al 2012;Bortolas et al 2016, and references therein).…”

Evolution of binary black holes in self gravitating discs

“…Additionally, it is still unclear how Nature bridges the gap between the two theoretically well understood stages of BHB evolution: (i) the dynamical friction driven stage, when the two BHs spiral in toward the centre of the merger remnant down to pc separations and (ii) the final inspiral driven by gravitational waves (GWs), which become efficient when the two BHs are at a separation < ∼ 10 −2 pc. Both dense stellar and gaseous environments have been shown to be effective in extracting the binary energy and angular momentum (see, e.g., Escala et al 2005;Dotti et al 2007;Cuadra et al 2009;Khan et al 2011;Preto et al 2011), likely driving the system to final coalescence (an extensive discussion on the fate of sub-parsec BHBs can be found in Dotti et al 2012). Scenarios involving cold gas are particular appealing not only A&A 545, A127 (2012) because they might produce distinctive observational signatures, but also because cold gas dominates the baryonic content in most galaxies at redshifts higher than one, providing a natural reservoir of energy and angular momentum to drive the BHB towards coalescence.…”

A Practical Guide to the Massive Black Hole Cosmic History

“…In gas-rich nuclei, torques exerted by a circumbinary disk can effectively extract the MBH binary energy and angular momentum, possibly resulting in a fast (∼10 7 yr) shrink to milliparsec separations [116,117], where gravitational wave (GW) emission efficiently drives its coalescence [118]. Also in gaspoor environments, recent simulations [119][120][121][122] showed that three body interactions with ambient stars efficiently fed (by rotation and triaxiality of the stellar distribution) into the binary loss cone can bring the system to final coalescence in ∼10 8 years. Along with the binary formation, during the merger, the cold gas content of the interacting system is highly destabilized, triggering inflows in the nuclear region [28,123] that provide a large reservoir of fuel for the active phase of the MBHs.…”

“…The work in [230] found that several hundred sources will contribute to the signal at a 1 ns level, considered the ultimate goal for the Square Kilometre Array (SKA [201]). Interestingly, such systems are far from coalescence, and they can still retain much of their original eccentricity against GW circularization [121,231,232]. Eccentricity measurements of individually resolved sources may help in constraining the evolution of MBH binaries, testing our current models of their dynamical evolution in star/gas dominated environments.…”