Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. II. Are Highly Obscured AGNs the Missing Link in the Merger-triggered AGN–Galaxy Coevolution Models?

By using a large, highly obscured (NH > 1023 cm-2) active galactic nucleus (AGN) sample (294 sources at z ∼ 0–5) selected from detailed X-ray spectral analyses in the deepest Chandra surveys, we explore distributions of these X-ray sources in various optical/infrared/X-ray color–color diagrams and their host-galaxy properties, aiming at characterizing the nuclear obscuration environment and the triggering mechanism of highly obscured AGNs. We find that the refined Infrared Array Camera (IRAC) color–color diagram fails to identify the majority of X-ray-selected, highly obscured AGNs, even for the most luminous sources with log LX (erg s-1) > 44. Over 80% of our sources will not be selected as heavily obscured candidates using the flux ratio of f24mm fR > 1000 and R − K > 4.5 criteria, implying complex origins and conditions for the obscuring materials that are responsible for the heavy X-ray obscuration. The average star formation rate (SFR) of highly obscured AGNs is similar to that of stellar mass- (M*-) and z-controlled normal galaxies, while a lack of quiescent hosts is observed for the former. Partial correlation analyses imply that highly obscured AGN activity (traced by LX) appears to be more fundamentally related to M*, and no dependence of NH on either M* or SFR is detected. Morphology analyses reveal that 61% of our sources have a significant disk component, while only ∼27% of them exhibit irregular morphological signatures. These findings together point toward a scenario where secular processes (e.g., galactic-disk instabilities), instead of mergers, are most probable to be the leading mechanism that triggers accretion activities of X-ray-selected, highly obscured AGNs.