A Systematic Analysis of Stellar Populations in the Host Galaxies of Changing-look AGNs

TL;DR

This study systematically analyzes the stellar populations and star formation histories of host galaxies in 26 changing-look AGNs, revealing similarities to normal AGNs and potential links between galaxy properties and CL phenomena.

Contribution

First comprehensive characterization of host galaxy stellar populations in CL-AGNs using stellar population synthesis, highlighting their similarities to normal AGNs and possible correlations with CL behavior.

Findings

CL-AGNs have similar stellar populations to normal AGNs, with more intermediate-age stars.

CL-AGNs follow the M_BH-σ_* relationship, consistent with other AGNs.

CL-AGNs with recent star formation tend to have higher Eddington ratios.

Abstract

"Changing-look" active galactic nuclei (CL-AGNs) are a newly-discovered class of AGNs that show the appearance (or disappearance) of broad emission lines within a short time scale (months to years) and are often associated with the dramatic change of their continuum emissions. They provide us an unprecedented chance to directly investigate the host galaxy properties with minimal contamination from the luminous central engine during the "turn-off" state, which is difficult for normal luminous AGNs. In this work, for the first time, we systematically characterize the stellar populations and star formation histories (SFHs) of host galaxies for 26 turn-off CL-AGNs using the stellar population synthesis code STARLIGHT. We find that the stellar populations of CL-AGNs are similar to that of normal AGNs, excepts that the intermediate stellar populations contribute more fraction. We estimate their stellar velocity dispersions ($\rm \sigma_{\star}$) and black hole masses ($\rm M_{BH,vir}$) and find that CL-AGNs also follow the overall $\rm M_{BH}-\sigma_{\star}$ relationship. We also confirm the previous claim that CL-AGNs tend to be biased towards lower Eddington ratio, and their extreme variabilities are more likely due to the intrinsic changes of accretion rates. In addition, CL-AGNs with recent star formations (SF) tend to have higher Eddington ratio. Compared with previous studies, our analysis suggests that there may be a correlation between the CL-AGN host galaxy properties and their CL phenomena.