Single and Binary Black Holes and Their Influence on Nuclear Structure

Massive central objects affect both the structure and evolution of galactic nuclei. Adiabatic growth of black holes generates power-law central density profiles with slopes in the range 1.5 <∼ −d log ρ/d log r <∼ 2.5, in good agreement with the profiles observed in the nuclei of galaxies fainter than MV ≈ −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 stellarand gas-dynamical processes at inducing coalescence of binary black holes, and uncoalesced binaries may be common in low-density nuclei. Numerical N -body experiments are not well suited to probing the long-term evolution of black hole binaries due to spurious relaxation.