Evolution of massive binary black holes

Since many or most galaxies have central massive black holes (BHs), mergers of galaxies can form massive binary black holes (BBHs). The evolution of BBHs depends on BH mass ratio and host galaxy type. BBHs with very low mass ratios (say, <~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 centre 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 <~ 90 km s^-1) or in highly flattened or triaxial galaxies. The semimajor axes of surviving BBHs are generally in the range 10-3-10 pc. The upper limit of this range is close to the HST resolution for the typical nearby galaxies (i.e., galaxies of the Virgo cluster). The absence of double nuclei in the centres of nearby galaxies does not necessarily mean that they have no BBHs. If all galaxies are highly triaxial, there will be no surviving BBHs. Possible observational characteristics of surviving BBHs are also discussed.