Galaxy Cores as Relics of Black Hole Mergers

We investigate the hypothesis that the cores of elliptical galaxies and bulges are created from the binding energy liberated by the coalescence of supermassive binary black holes during galaxy mergers. Assuming that the central density profiles of galaxies were initially steep power laws, $\rho\sim r^{-2}$, we define the ``mass deficit'' as the mass in stars that had to be removed from the nucleus in order to produce the observed core. We use nonparametric deprojection to compute the mass deficit in a sample of 35 early-type galaxies with high-resolution imaging data. We find that the mass deficit correlates well with the mass of the nuclear black hole, consistent with the predictions of merger models. We argue that the level of damage done to stellar cusps by merging black holes is a lower limit on the damage expected for cusps of non-interacting dark matter.