The hidden "AGN main sequence": Evidence for a universal black hole accretion to star formation rate ratio since z~2 producing a M_BH-M* relation

TL;DR

This study shows that supermassive black hole growth and star formation in galaxies have been closely linked since z~2, maintaining a consistent ratio and suggesting common underlying gas-driven processes.

Contribution

It provides evidence for a universal black hole accretion to star formation rate ratio since z~2, indicating synchronized growth of SMBHs and their host galaxies.

Findings

SMBH growth rate correlates with galaxy stellar mass similarly to SFR.

The SMBH to SFR ratio remains constant across different masses and redshifts.

SMBH and star formation growth are driven by common gas reservoir availability.

Abstract

Using X-ray stacking analyses we estimate the average amounts of supermassive black hole (SMBH) growth taking place in star-forming galaxies (SFGs) at z~1 and z~2 as a function of galaxy stellar mass (M*). We find the average SMBH growth rate follows remarkably similar trends with M* and redshift as the average star-formation rates (SFRs) of their host galaxies (i.e., (i.e., dM_BH/dt ~ M*^(0.86+/-0.39) for the z~1 sample and dM_BH/dt ~ M*^(1.05+/-0.36) for the z~2 sample). It follows that the ratio of SMBH growth rate to SFR is (a) flat with respect to M* (b) not evolving with redshift and (c) close to the ratio required to maintain/establish a SMBH to M* ratio of ~10^{-3} as also inferred from today's M_BH-M_bulge relationship. We interpret this as evidence that SMBHs have, on average, grown in-step with their host galaxies since at least z~2, irrespective of host galaxy mass and AGN triggering mechanism. As such, we suggest that the same secular processes that drive the bulk of star formation are also responsible for the majority of SMBH growth. From this, we speculate that it is the availability of gas reservoirs that regulate both cosmological SMBH growth and star formation.