Diverse pathways for supermassive black hole-galaxy coevolution

TL;DR

This paper develops an empirical model using observational data to explore the diverse coevolutionary pathways of supermassive black holes and their host galaxies from redshift 2 to 0, revealing varied growth histories and evolution of their mass relations.

Contribution

It introduces a novel empirical approach combining observed accretion rates and galaxy properties to trace SMBH and galaxy coevolution across cosmic time.

Findings

Massive SMBHs grow mostly before z=2, indicating early assembly.

Lower mass SMBHs gradually grow from z=2 to 0.

The SMBH-stellar mass relation evolves significantly with redshift.

Abstract

Supermassive black holes (SMBHs) are observed in diverse galaxy populations across time yet a clear understanding of how they coevolve with their hosts has not been reached. Physically-motivated models of SMBH accretion and feedback vary widely between galaxy formation simulations due to the difficulty of modeling the range of scales important for galactic and SMBH processes. Here we use observational data to build an empirical model for SMBH growth. We apply observed specific accretion rate probability distributions as a function of galaxy star formation rate between $z = 0-2$ to the UniverseMachine galaxy formation model to determine SMBH accretion rates based on galaxy properties. We use observed $z = 0$ SMBH-stellar mass relations for the quiescent and star-forming populations to provide the local boundary conditions for SMBH growth histories. We then track the coevolutionary histories of galaxy stellar mass and their SMBHs backwards in time to $z = 2$. We find that the most massive SMBHs at $z = 0$ have grown very little of their total mass between $z = 0-2$, indicating early SMBH mass assembly for these systems. Conversely, lower mass SMBHs at $z = 0$ assembled their mass gradually across $z = 0-2$. This results in substantial evolution of the SMBH-stellar mass relation, shifting to higher normalization and shallower slope with increasing redshift. We find that the substantial scatter observed in the $z = 0$ SMBH-stellar mass relation results in the diversity of growth pathways found in our model, with some galaxies assembling their stellar mass before their SMBHs and others doing the opposite.