  Massive central objects affect both the structure and evolution of galactic
nuclei. Adiabatic growth of black holes generates power-law central density
profiles with logarithmic slopes in the range from ~1.5 to ~2.5, in good
agreement with the profiles observed in the nuclei of galaxies fainter than
visual magnitude -20. However the shallow nuclear profiles of bright galaxies
require a different explanation. Binary black holes are an inevitable result of
galactic mergers, and the ejection of stars by a massive binary displaces a
mass of order the binary's own mass, creating a core or shallow power-law cusp.
This model is at least crudely consistent with core sizes in bright galaxies.
Uncertainties remain about the effectiveness of stellar- and gas-dynamical
processes at inducing coalescence of binary black holes, and uncoalesced
binaries may be common in low-density nuclei.
