2024ApJ...969...11H

We present the study on the relationship between supermassive black holes (SMBHs) and their host galaxies using our variability-selected active galactic nuclei (AGNs) sample (i_AB ≤ 25.9 and z ≤ 4.5) constructed from the Hyper Suprime-Cam Subaru Strategic Program Ultradeep survey in the COSMOS field. We estimated the black hole (BH) mass (M_BH = 10^5.5–10M_⊙) based on the single-epoch virial method and the total stellar mass (M_star = 10^10–12M_⊙) by separating the AGN component with spectral energy distribution fitting. We found that the redshift evolution of the BH-stellar mass ratio (M_BH/M_star) depends on the M_BH, which is caused by no significant correlation between M_BH and M_star. Variable AGNs with massive SMBHs (M_BH > 10⁹M_⊙) at 1.5 < z < 3 show considerably higher BH-stellar mass ratios (> ∼1%) than the BH-bulge ratios (M_BH/M_bulge) observed in the local Universe for the same BH range. This implies that there is a typical growth path of massive SMBHs, which is faster than the formation of the bulge component as final products seen in the present day. For the low-mass SMBHs (M_BH < 10⁸M_⊙) at 0.5 < z < 3, on the other hand, variable AGNs show similar BH-stellar mass ratios with the local objects (∼0.1%), but smaller than those observed at z > 4. We interpret that host galaxies harboring less massive SMBHs at intermediate redshift have already acquired sufficient stellar mass, although high-z galaxies are still in the early stage of galaxy formation relative to those at the intermediate/local Universe.