Evidence for Merger-Driven Growth in Luminous, High-z, Obscured AGN in the CANDELS/COSMOS Field

TL;DR

This study provides evidence that major mergers are a significant mechanism for fueling high-luminosity, heavily obscured AGN at high redshift, contrasting with lower-luminosity AGN where secular processes dominate.

Contribution

It demonstrates that high-luminosity, obscured AGN are more likely to be merger-driven, based on morphological analysis in the CANDELS/COSMOS field, highlighting the role of mergers in their growth.

Findings

IR-only AGN are more irregular and asymmetric than X-ray-only AGN.

IR-only AGN are less likely to have spheroidal host components.

A higher fraction of IR-only AGN show signs of interactions or mergers.

Abstract

While major mergers have long been proposed as a driver of both AGN activity and the M-sigma relation, studies of moderate to high redshift Seyfert-luminosity AGN hosts have found little evidence for enhanced rates of interactions. However, both theory and observation suggest that while these AGN may be fueled by stochastic accretion and secular processes, high-luminosity, high-redshift, and heavily obscured AGN are the AGN most likely to be merger-driven. To better sample this population of AGN, we turn to infrared selection in the CANDELS/COSMOS field. Compared to their lower-luminosity and less obscured X-ray-only counterparts, IR-only AGN (luminous, heavily obscured AGN) are more likely to be classified as either irregular (50$^{+12}_{-12}$% vs. 9$^{+5}_{-2}$%) or asymmetric (69$^{+9}_{-13}$% vs. 17$^{+6}_{-4}$%) and are less likely to have a spheroidal component (31$^{+13}_{-9}$% vs. 77$^{+4}_{-6}$%). Furthermore, IR-only AGN are also significantly more likely than X-ray-only AGN (75$^{+8}_{-13}$% vs. 31$^{+6}_{-6}$%) to be classified either as interacting or merging in a way that significantly disturbs the host galaxy or disturbed though not clearly interacting or merging, which potentially represents the late stages of a major merger. This suggests that while major mergers may not contribute significantly to the fueling of Seyfert luminosity AGN, interactions appear to play a more dominant role in the triggering and fueling of high-luminosity heavily obscured AGN.