Core depletion from coalescing supermassive black holes
  New measurements of the stellar-mass deficits at the centers of luminous elliptical galaxies are presented. These were derived considering the following observational facts. Firstly, ``core'' galaxies, which are thought to have had their inner region depleted from the coalesence of supermassive black holes, show an abrupt downward deviation of their inner light-profile relative to their outer Sersic profile. Second, ``power-law'' galaxies, having undisturbed profiles and no partially depleted core, have inner light-profiles that display no departure from the inward extrapolation of their outer Sersic profile. The central stellar deficits have therefore been derived from the difference in flux between the HST-observed galaxy light-profiles and the inward extrapolation of each galaxy's outer Sersic profile. This approach gives flux deficits ~0.1% of the total galaxy light, and mass deficits that are ~2 times each galaxy's central supermassive black hole mass. These results are in agreement with the theoretical expectations of mass ejection from binary black hole mergers and also with popular LCDM models of hierarchical galaxy formation. It is also explained why this result is some 10 times smaller than current observational estimates of the central mass deficit, and therefore implies a merger history for giant elliptical galaxies that is one order of magnitude less violent than previously suggested. 
Alister W. Graham