Evident black hole-bulge coevolution in the distant universe

Abstract

ABSTRACT Observations in the local universe show a tight correlation between the masses of supermassive black holes (SMBHs; MBH) and host-galaxy bulges (Mbulge), 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 ($\overline{\rm BHAR}$) for bulge-dominated galaxies at z = 0.5–3. While previous observations found $\overline{\rm BHAR}$ is strongly related to host-galaxy stellar mass (M⋆) for the overall galaxy population, our analyses show that, for the bulge-dominated population, $\overline{\rm BHAR}$ is mainly related to SFR rather than M⋆. This ${\overline{\rm BHAR}}$–SFR relation is highly significant, e.g. 9.0σ (Pearson statistic) at z = 0.5–1.5. Such a $\overline{\rm BHAR}$–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 MBH−Mbulge correlation. Our best-fitting ${\overline{\rm BHAR}}$–SFR relation for the bulge-dominated sample is ${\log \overline{\rm BHAR}= \log \mathrm{SFR} - (2.48\pm 0.05)}$ (solar units). The best-fitting $\overline{\rm BHAR}/\mathrm{SFR}$ ratio (10−2.48) for bulge-dominated galaxies is similar to the observed MBH/Mbulge 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 MBH−Mbulge correlation. This lockstep growth also predicts that the MBH−Mbulge relation should not have strong redshift dependence.