The First Massive Black Hole Seeds and Their Hosts

TL;DR

This study uses cosmological simulations to explore how the first massive black holes form in high-redshift galaxies, revealing their dependence on halo mass, redshift, and formation efficiency, and explaining their presence in various galaxy types.

Contribution

It introduces a novel simulation-based model for seed black hole formation based on local gas conditions, advancing understanding of black hole-host galaxy relationships.

Findings

Black hole seeds form in halos of 10^7 to 10^9 Msun.

Formation rate drops around redshift 5 due to metal diffusion.

By redshift 5, larger halos almost always host black holes.

Abstract

We investigate the formation of the first massive black holes in high redshift galaxies, with the goal of providing insights to which galaxies do or do not host massive black holes. We adopt a novel approach to forming seed black holes in galaxy halos in cosmological SPH+N-body simulations. The formation of massive black hole seeds is dictated directly by the local gas density, temperature, and metallicity, and motivated by physical models of massive black hole formation. We explore seed black hole populations as a function of halo mass and redshift, and examine how varying the efficiency of massive black hole seed formation affects the relationship between black holes and their hosts. Seed black holes tend to form in halos with mass between 10^7 and 10^9 Msun, and the formation rate is suppressed around z = 5 due to the diffusion of metals throughout the intergalactic medium. We find that the time of massive black hole formation and the occupation fraction of black holes are a function of the host halo mass. By z = 5, halos with mass M_halo > 3 x 10^9 Msun host massive black holes regardless of the efficiency of seed formation, while the occupation fraction for smaller halos increases with black hole formation efficiency. Our simulations explain why massive black holes are found in some bulgeless and dwarf galaxies, but we also predict that their occurrence becomes rarer and rarer in low-mass systems.