Evident black hole-bulge coevolution in the distant universe

Abstract

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 (\overlineBHAR) for bulge-dominated galaxies at z = 0.5-3. While previous observations found \overlineBHAR is strongly related to host-galaxy stellar mass (M_⋆) for the overall galaxy population, our analyses show that, for the bulge-dominated population, \overlineBHAR is mainly related to SFR rather than M_⋆. This {\overlineBHAR}-SFR relation is highly significant, e.g. 9.0σ (Pearson statistic) at z = 0.5-1.5. Such a \overlineBHAR-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 {\overlineBHAR}-SFR relation for the bulge-dominated sample is {log \overlineBHAR= log SFR - (2.48± 0.05)} (solar units). The best-fitting \overlineBHAR/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.