Formation of Galactic Nuclei

We investigate a model in which galactic nuclei form via the coalescence ofpre-existing stellar systems containing supermassive black holes. Mergersimulations are carried out using N-body algorithms that can follow theformation and decay of a black-hole binary and its effect on the surroundingstars down to sub-parsec scales. Our initial stellar systems have steep centraldensity cusps similar to those in low-luminosity elliptical galaxies. Formationof a black-hole binary transfers energy to the stars and lowers the centraldensity; continued decay of the binary creates a ~1/r density cusp similar tothose observed in bright elliptical galaxies, with a break radius that extendswell beyond the sphere of gravitational influence of the black holes. The decayof the black hole binary is followed over a factor of ~20 in separation afterformation of a hard binary, considerably farther than in previous simulations.We see almost no dependence of the binary's decay rate on number of particlesin the simulation, contrary to earlier studies in which a lower initial densityof stars led to a more rapid depletion of the binary's loss cone. Wenevertheless argue that the decay of a black hole binary in a real galaxy wouldbe expected to stall at separations of 0.01-1 pc unless some additionalmechanism is able to extract energy from the binary. Our results support apicture in which the observed dependence of nuclear cusp slope on galaxyluminosity is a consequence of galaxy interactions. We also discuss theimplications of our results for the survivability of dark-matter cusps.Comment: 35 pages, 19 postscript figures, uses emulateapj.sty, onecolfloat.st