The formation and evolution of massive black hole seeds in the early Universe

TL;DR

This study models the formation and evolution of massive black hole seeds from direct gas collapse at high redshifts, predicting their observable signatures and differences from light seed models in galaxy properties and black hole distributions.

Contribution

It introduces a formalism to track massive seed black hole evolution and compares predictions with light seed models, highlighting differences in low mass galaxy black hole occupation.

Findings

Massive seed models predict fewer black holes in low mass, bulge-less galaxies.

Predicted low-mass end of the M-sigma relation matches recent observations.

Significant differences between models are mainly at the low mass end of the black hole mass function.

Abstract

Tracking the evolution of high redshift seed black hole masses to late times, we examine the observable signatures today. These massive initial black hole seeds form at extremely high redshifts from the direct collapse of pre-galactic gas discs. Populating dark matter halos with seeds formed in this fashion, we follow the mass assembly history of these black holes to the present time using a Monte-Carlo merger tree approach. Utilizing this formalism, we predict the black hole mass function at high redshifts and 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 M-sigma relation. Comparing the predictions of the `light' seed model with these massive seeds we find that significant differences appear predominantly at the low mass end of the present day black hole mass function. However, all our 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 has a direct influence on the black hole occupation fraction in galaxies at z=0. This effect is more pronounced for low mass galaxies. This is the key discriminant between the models studied here and the Population III remnant `light' seed model. We find that there exists a population of low mass galaxies that do not host nuclear black holes. Our prediction of the shape of the M-sigma relation at the low mass end and increased scatter has recently been corroborated by observations.