Journey to the M_BH -sigma relation: the fate of low mass black holes in the Universe

TL;DR

This study investigates how the black hole mass-velocity dispersion relation forms and evolves over cosmic time, considering different seed models and accretion processes, predicting observable signatures and hidden populations.

Contribution

It introduces models of black hole seed formation and tracks their evolution, revealing how the $M_{BH}-\sigma$ relation develops and depends on initial conditions and merger-driven growth.

Findings

Massive black hole seeds influence the low-mass end of the relation.

Major mergers are key to establishing and maintaining the $M_{BH}-\sigma$ relation.

A large population of undetectable low-mass black holes is predicted at high redshift.

Abstract

In this paper, we explore the establishment and evolution of the empirical correlation between black hole mass and velocity dispersion with redshift. We track the growth and accretion history of massive black holes starting from high redshift using two seeding models:(i) Population III remnants, and (ii) massive seeds from direct gas collapse. Although the seeds do not initially satisfy the $M_{\rm BH} - \sigma$ relation, the correlation is established and maintained at all times if self-regulating accretion episodes are associated with major mergers. The massive end of the $M_{\rm BH} - \sigma$ relation is established early, and lower mass MBHs migrate over time. How MBHs migrate toward the relation, the slope and the scatter of the relation all depend critically on the seeding model as well as the adopted self-regulation prescription. We expect flux limited AGN surveys and LISA to select accreting and merging MBHs respectively that have already migrated onto the $\msigma$ relation. This is a consequence of major mergers being more common at high redshift for the most massive, biased, galaxies that anchor the $\msigma$ relation early. We also predict the existence of a large population of low mass `hidden' MBHs at high redshift which can easily escape detection. Additionally, we find that if MBH seeds are massive, $\sim 10^5 M_{\odot}$, the low-mass end of the $\msigma$ flattens towards this asymptotic value, creating a characteristic `plume'.