A Morphological Study on Galaxies Hosting Optical Variability-Selected AGNs in the COSMOS Field

TL;DR

This study investigates the morphology of host galaxies of optical variability-selected low luminosity AGNs in the COSMOS field, revealing diverse galaxy types and connections between star formation, mergers, and AGN activity.

Contribution

It provides new morphological measurements of AGN hosts at high redshift using HST data, highlighting the diversity of host galaxy types and their relation to AGN properties.

Findings

Most AGN hosts are compact point-like sources.

Merger rate correlates with star formation and AGN luminosity.

Optical variability-selected AGNs are more likely in elliptical galaxies.

Abstract

The morphological study is crucial to investigate the connections between active galactic nuclei (AGN) activities and the evolution of galaxies. Substantial studies have found that radiative-mode AGNs primarily reside in disk galaxies, questioning the merger-driven mechanism of AGN activities. In this study, through S{\'e}rsic profile fitting and non-parametric morphological parameter measurements, we investigated the morphology of host galaxies of 485 optical variability-selected low luminosity AGNs at $z\lesssim4.26$ in the COSMOS field. We analyzed high-resolution images of the Hubble Space Telescope to measure these morphological parameters. We only successfully measured the morphological parameters for 76 objects and most AGN hosts ($\sim70\%$) were visually compact point-like sources. We examined the obtained morphological information as a function of redshift and compared them with literature data. We found that these AGN host galaxies showed no clear morphological preference. However, the merger rate increased with the higher hosts' SFRs and AGN luminosity. Interestingly, we found ongoing star formation consistent with the typical star forming populations in both elliptical and spiral galaxies while these two types of galaxies were more symmetric than normal star forming galaxies. These results suggested that optical variability-selected AGNs have} higher probabilities to reside in elliptical galaxies than infrared-selected AGNs (IR-AGNs), whose host galaxies had a strong disk-dominance, and supported recent studies that the AGN feedback could enhance star forming activities in host galaxies.