Multiple black holes in galactic bulges
  We study the number and interaction rates of supermassive black holes in galactic bulges as predicted by hierarchical models of galaxy formation in which the spheroidal components of galaxies are formed by mergers. In bright ellipticals, the number of events that can eject a central supermassive binary black hole is large. Central binaries must therefore merge in less than a Hubble time -- otherwise there will be too much scatter in the M-sigma relation and too many off-center galactic nuclei. We propose that binary black holes are able to merge during the major gas accretion events that trigger QSO activity in galaxies. If this is the case, less than 10 percent of faint ellipticals and 40 percent of bright ellipticals are predicted to harbour binary black holes with near equal masses at their centres. This binary may be ejected away from the centre of the galaxy or even into intergalactic space in up to 20 percent of the most luminous ellipticals. The number of low mass black holes that can interact with the central object is predicted to be a strong function of galaxy luminosity. In most faint ellipticals, no black holes fall into the centre of the galaxy after the last major gas accretion event, but in the most luminous ellipticals, an average of 10 low mass black holes interact with the central supermassive object after this time. It is expected that stars will be ejected from galaxy cores as these low mass-ratio binaries harden. Multiple black holes in galactic bulges thus provide a natural explanation for the strong systematic trends in the observed central density profiles of ellipticals as a function of luminosity. 
Martin G. Haehnelt (IoA) and Guinevere Kauffmann (MPA)