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?

Abstract

By using a large, highly obscured ( ${N}_{{\rm{H}}}\gt {10}^{23}\ {\mathrm{cm}}^{-2}$ ) 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 $\mathrm{log}\,{L}_{{\rm{X}}}(\mathrm{erg}\ {{\rm{s}}}^{-1})\gt 44$ . Over 80% of our sources will not be selected as heavily obscured candidates using the flux ratio of ${f}_{24\mu {\rm{m}}}/{f}_{R}\gt 1000$ 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 ${L}_{{\rm{X}}}$ ) appears to be more fundamentally related to M_*, and no dependence of ${N}_{{\rm{H}}}$ 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.