There are supermassive black holes (SMBHs) in the centers of most massive galaxies. Observations of nearby systems have found that SMBH masses (M_BH) are tightly correlated with host-galaxy properties such as bulge masses (M_bulge). These local SMBH-galaxy relations suggest that SMBH growth is fundamentally linked to host galaxies over cosmic history. Previous studies suggest that long-term average SMBH accretion rate (BHAR) is intrinsically related to star formation rate (SFR) for the overall galaxy population. However, we show that BHAR is more strongly correlated with host-galaxy stellar mass (M_*) rather than SFR, and this BHAR-M_* relation does not depend on cosmic environment. We further quantify this BHAR-M_* relation and its cosmic evolution at z= 0.4–4. 

 

However, we find this BHAR-M_* relation does not hold for bulge-dominated galaxies, and their BHAR primarily depends on SFR. This BHAR-bulge SFR relation indicates that SMBHs only coevolve with galactic bulges rather than the entire galaxies, consistent with the observations of the local universe. Our best-fit BHAR/SFR ratio is similar to the typical M_BH/M_bulge ratio observed in the local systems, indicating that the BHAR-bulge SFR relation is indeed responsible for the local M_BH-M_bulge relation. Our high-resolution hydrodynamical simulations suggest that a compact circum-nuclear disk is the physical mechanism that drives the BHAR-bulge SFR relation. Our recent ALMA observations support this idea. 

 

We have recently applied the BHAR-bulge SFR relation to track cosmic BH accretion density (BHAD) based on the measurements of bulge star-formation history (SFH) from Hubble grism data. Our derived BHAD at z>4 is much higher than those observed from X-ray surveys, indicating that a large population of Compton-thick AGNs is missing from X-ray observations. Future JWST deep surveys will be able to identify these Compton-thick AGNs.