AGN Host Galaxy Morphologies in COSMOS

TL;DR

This study analyzes the morphologies of approximately 400 AGN host galaxies in the COSMOS field using HST imaging, revealing a diverse range of structures and suggesting that major mergers are not the primary trigger for AGN activity at these luminosities.

Contribution

It provides a detailed morphological analysis of X-ray selected AGN hosts, including the effects of nuclear point sources, and compares them with non-active galaxies to understand their evolutionary state.

Findings

AGN hosts span from early- to late-type morphologies.

No excess of companion galaxies around AGN, indicating no major mergers.

Host galaxies may be transitioning from disk- to bulge-dominated without major mergers.

Abstract

We use HST/ACS images and a photometric catalog of the COSMOS field to analyze morphologies of the host galaxies of approximately 400 AGN candidates at redshifts 0.3 < z < 1.0. We compare the AGN hosts with a sample of non-active galaxies drawn from the COSMOS field to match the magnitude and redshift distribution of the AGN hosts. We perform 2-D surface brightness modeling with GALFIT to yield host galaxy and nuclear point source magnitudes. X-ray selected AGN host galaxy morphologies span a substantial range that peaks between those of early-type, bulge-dominated and late-type, disk-dominated systems. We also measure the asymmetry and concentration of the host galaxies. Unaccounted for, the nuclear point source can significantly bias results of these measured structural parameters, so we subtract the best-fit point source component to obtain images of the underlying host galaxies. Our concentration measurements reinforce the findings of our 2-D morphology fits, placing X-ray AGN hosts between early- and late-type inactive galaxies. AGN host asymmetry distributions are consistent with those of control galaxies. Combined with a lack of excess companion galaxies around AGN, the asymmetry distributions indicate that strong interactions are no more prevalent among AGN than normal galaxies. In light of recent work, these results suggest that the host galaxies of AGN at these X-ray luminosities may be in a transition from disk-dominated to bulge-dominated, but that this transition is not typically triggered by major mergers.