Evolution of massive binary black holes

Abstract

Since many or most galaxies have central massive black holes (BHs), mergers of galaxies can form massive binary black holes (BBHs). In this paper, we study the evolution of massive BBHs in realistic galaxy models, using a generalization of techniques used to study tidal disruption rates around massive BHs. The evolution of BBHs depends on BH mass ratio and host galaxy type. BBHs with very low mass ratios (say, $\la$ 0.001) are hardly ever formed by mergers of galaxies because the dynamical friction timescale is too long for the smaller BH to sink into the galactic center within a Hubble time. BBHs with moderate mass ratios are most likely to form and survive in spherical or nearly spherical galaxies and in high-luminosity or high-dispersion galaxies; they are most likely to have merged in low-dispersion galaxies (line-of-sight velocity dispersion $\la$ 90 km/s) or in highly flattened or triaxial galaxies. The semimajor axes and orbital periods of surviving BBHs are generally in the range 10^{-3}-10 pc and 10-10^5 yr; and they are larger in high-dispersion galaxies than in low-dispersion galaxies, larger in nearly spherical galaxies than in highly flattened or triaxial galaxies, and larger for BBHs with equal masses than for BBHs with unequal masses. The orbital velocities of surviving BBHs are generally in the range 10^2-10^4 km/s. The methods of detecting surviving BBHs are also discussed. If no evidence of BBHs is found in AGNs, this may be either because gas plays a major role in BBH orbital decay or because nuclear activity switches on soon after a galaxy merger, and ends before the smaller BH has had time to spiral to the center of the galaxy.Comment: 32 pages, including 14 figures, submitted to MNRA

Efficient Merger of Binary Supermassive Black Holes in Nonaxisymmetric Galaxies

“…N-body simulations (Makino & Funato 2004;Szell et al 2005;Berczik et al 2005) show that continued hardening of the binary takes place at a rate that depends strongly on the number N of "star" particles used in the simulation. As N increases, the hardening rate falls, as expected if the binary's loss cone is repopulated by star-star gravitational encounters (Yu 2002;Milosavljević & Merritt 2003). When extrapolated to the much larger N of real galaxies, these results suggest that binary evolution would generally stall (the "final-parsec problem").…”

Radio structure of the blazar 1156 + 295 with sub-pc resolution

“…Yu 2002) show that the orbital parameters of a survival BBH depend on the galactic velocity dispersions and blackhole mass ratios. An orbital period of a few tens of years suggests a mass ratio of the secondary to primary black holes of between 0.01 and 0.05.…”

“…Combining the close separation of the binaries and the relatively low mass of the secondary BH, the radio emission from the secondary BH might be very weak and below our current detectable limit in VLBI. Evidence of possible BBHs in 1156+295 may be represented by possible periodic variabilities in the radio, optical, and X-ray light curves, and the expected apparent variability timescale would be suppressed by relativistic aberration resulting in an orbital period much shorter than the intrinsic value (Yu 2002).…”

Untitled

“…The effect of loss-cone depletion (the depletion of low angular momentum stars that get close enough to extract energy from a hard binary) has traditionally been one of the major uncertainties in constructing viable merger scenarios for MBH binaries ( Begelman et al 1980). Several processes have recently been studied that should all mitigate the problems associated with loss-cone depletion, such as the wandering of the binary center of mass from the galaxy center induced by continuous interactions with background stars (Chatterjee et al 2003), the large supply of low angular momentum stars in significantly flattened or triaxial galaxies ( Yu 2002), the presence of a third hole ( Blaes et al 2002), and the randomization of stellar orbits due to the infall of small satellites (Zhao et al 2002).…”