The mass assembly history of black holes in the Universe

Abstract: We track the growth and evolution of high redshift seed black holes over cosmic time. This population of massive, initial black hole seeds form at these early epochs from the direct collapse of pre-galactic gas discs. Populating dark matter halos with seeds formed in this fashion, we follow their mass assembly history to the present time using a Monte-Carlo merger tree approach. Using this formalism, we predict the black hole mass function at the present time; the integrated mass density of black holes in the Universe; the luminosity function of accreting black holes as a function of redshift and the scatter in observed, local Mbh{\sigma}s relation. Signatures of these massive seed models appear predominantly at the low mass end of the present day black hole mass function. In fact, our prediction of the shape of the Mbh{\sigma}s relation at the low mass end and increased scatter has recently been corroborated by observations. These models predict that low surface brightness, bulge-less galaxies with large discs are least likely to be sites for the formation of massive seed black holes at high redshifts. The efficiency of seed formation at high redshifts also has a direct influence on the black hole occupation fraction in galaxies at z = 0. This effect is more pronounced for low mass galaxies today as we predict the existence of a population of low mass galaxies that do not host nuclear black holes. This is the key discriminant between the models studied here and the Population-III remnant seed model.