Diverse Pathways for Supermassive Black Hole–Galaxy Coevolution

Abstract Supermassive black holes (SMBHs) are observed in diverse galaxy populations across cosmic time, yet a clear understanding of how they coevolve with their hosts has not been reached. Physical 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 star formation rate between z = 0 and 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 SMBH and galaxy stellar mass backward in time to z = 2. We find that the most massive SMBHs at z = 0 have grown very little of their mass between z = 0 and 2, indicating early SMBH mass assembly for these systems. Conversely, lower-mass SMBHs at z = 0 assembled 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.