The relationship of SMBHs and host galaxies at $z<4$ in the deep optical variability-selected AGN sample in the COSMOS field

TL;DR

This study investigates the evolution of the relationship between supermassive black holes and their host galaxies up to redshift 4, revealing different growth patterns for SMBHs of varying masses over cosmic time.

Contribution

It provides new insights into SMBH and galaxy co-evolution by analyzing a large variability-selected AGN sample with detailed mass estimates across a wide redshift range.

Findings

Massive SMBHs at 1.5<z<3 have higher BH-stellar mass ratios than local bulge ratios.

Low-mass SMBHs at 0.5<z<3 have similar ratios to local objects, but lower than at z>4.

The growth of massive SMBHs outpaces bulge formation, indicating different evolutionary paths.

Abstract

We present the study on the relationship between SMBHs and their host galaxies using our variability-selected AGN sample ($i_\mathrm{AB} \leq 25.9,\ z \leq 4.5$) constructed from the HSC-SSP Ultra-deep survey in the COSMOS field. We estimated the BH mass ($M_\mathrm{BH}=10^{5.5-10}\ M_{\odot}$) based on the single-epoch virial method and the total stellar mass ($M_\mathrm{star}=10^{10-12}\ M_{\odot}$) by separating the AGN component with SED fitting. We found that the redshift evolution of the BH-stellar mass ratio ($M_\mathrm{BH}/M_\mathrm{star}$) depends on the $M_\mathrm{BH}$ which is caused by the no significant correlation between $M_\mathrm{BH}$ and $M_\mathrm{star}$. Variable AGNs with massive SMBHs ($M_\mathrm{BH}>10^{9}\ M_{\odot}$) at $1.5<z<3$ show considerably higher BH-stellar mass ratios ($>\sim1\%$) than the BH-bulge ratios ($M_\mathrm{BH}/M_\mathrm{bulge}$) 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 ($M_\mathrm{BH}<10^{8}\ M_{\odot}$) at $0.5<z<3$, on the other hand, variable AGNs show the similar BH-stellar mass ratios with the local objects ($\sim 0.1\%$) 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.