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 H > 10 23 cm −2 ) 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/IR/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 IRAC color-color diagram fails to identify the majority of X-ray selected highly obscured AGNs, even for the most luminous sources with log L X (erg s −1 ) > 44. Over 80% of our sources will not be selected as heavily obscured candidates using the flux ratio of f 24µm /f R > 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 of highly obscured AGNs is similar to that of stellar mass-(M * -) and z-controlled normal galaxies, while the lack of quiescent hosts is observed for the former. Partial correlation analyses imply that highly obscured AGN activity (traced by L X ) appears to be more fundamentally related to M * , and no dependence of N 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.

The Sizes of Quasar Host Galaxies with the Hyper Suprime-Cam Subaru Strategic Program

“…Since mergerinduced size growth is most effective for gas-poor galaxies (i.e., dry mergers), and our quasars which are still actively forming stars have not yet evolved to such a stage, the resulting growth lies in the galaxies' future. Combining structural analyses for various AGN populations in different stages of evolution, such as heavily obscured AGNs which may represent an early fast growth phase of SMBHs (Kocevski et al 2015;Li et al 2020), post-starburst quasars which connect quasars to recently quenched galaxies (e.g., Cales & Brotherton 2015;Matsuoka et al 2015), radio AGNs which represent the low state of black hole accretion (e.g., Best et al 2005), and galaxies showing past AGN activity like those with extended emission line regions (e.g, Keel et al 2012;Ichikawa et al 2019a,b) may complete the evolutionary path and shed light on how AGN activity may affect galaxy evolution.…”

“…Such a connection could reveal the role that galaxy structures play in triggering gas inflows. Studies of X-ray and opticallyselected AGNs show that the majority reside in disk galaxies, possibly indicating that structures, such as bars or nuclear spirals, play a role in transporting cold gas to the nucleus (e.g., Gabor et al 2009;Schawinski et al 2011;Kocevski et al 2012;Yue et al 2018;Zhao et al 2019;Li et al 2020). There is currently much debate as to whether major mergers, capable of efficiently driving gas inwards through tidally induced torques (e.g., Hopkins et al 2008), are the driver of luminous AGN activity (e.g., Villforth et al 2014;Mechtley et al 2016;Villforth et al 2017;Goulding et al 2018;Marian et al 2019Marian et al , 2020.…”