Evident black hole-bulge coevolution in the distant universe

Observations in the local universe show a tight correlation between the masses of supermassive black holes (SMBHs; M_BH) and host-galaxy bulges (M_bulge), suggesting a strong connection between SMBH and bulge growth.However, direct evidence for such a connection in the distant universe remains elusive. We have studied sample-averaged SMBH accretion rate (BHAR) for bulge-dominated galaxies at z = 0.5–3. While previous observations found BHAR is strongly related to host-galaxy stellar mass (M_*) for the overall galaxy population, our analyses show that, for the bulge-dominated population, BHAR is mainly related to SFR rather than M_*. This BHAR–SFR relation is highly significant, e.g. 9.0σ (Pearson statistic) at z = 0.5–1.5. Such a BHAR–SFR connection does not exist among our comparison sample of galaxies that are not bulge dominated, for which M_* appears to be the main determinant of SMBH accretion. This difference between the bulge-dominated and comparison samples indicates that SMBHs only coevolve with bulges rather than the entire galaxies, explaining the tightness of the local M_BH−M_bulge correlation. Our best-fitting BHAR–SFR relation for the bulge-dominated sample is log BHAR = log SFR − (2.48 ± 0.05) (solar units). The best-fitting BHAR/SFR ratio (10^−2.48) for bulge-dominated galaxies is similar to the observed M_BH/M_bulge values in the local universe. Our results reveal that SMBH and bulge growth are in lockstep, and thus non-causal scenarios of merger averaging are unlikely the origin of the M_BH−M_bulge correlation. This lockstep growth also predicts that the M_BH−M_bulge relation should not have strong redshift dependence.