[O IV]- and [Ne V]-weak Active Galactic Nuclei Hidden by Compton-thick Material in Late Mergers

Abstract

We study "buried" active galactic nuclei (AGNs) almost fully covered by circumnuclear material in ultra/luminous infrared galaxies (U/LIRGs), which show weak ionized lines from narrow-line regions. Employing an indicator of a [O IV] 25.89 or [Ne V] 14.32 μm line to 12 μm AGN luminosity ratio, we find 17 buried AGN candidates that are [O IV]-weak (L _{[O IV]}/L _12,AGN ≤ ‑3.0) or [Ne V]-weak (L _{[Ne V]}/L _12,AGN ≤ ‑3.4) among 30 AGNs in local U/LIRGs. For the [O IV]-weak AGNs, we estimate their covering fractions of Compton-thick (CT; N _H ≥ 10²⁴ cm^‑2) material with an X-ray clumpy torus model to be ${f}_{\mathrm{CT}}^{(\mathrm{spec})}=0.55\pm 0.19$ on average. This value is consistent with the fraction of CT AGNs ( ${f}_{\mathrm{CT}}^{(\mathrm{stat})}=53 \% \pm 12 \% $ ) among the [O IV]-weak AGNs in U/LIRGs and much larger than that in Swift/Burst Alert Telescope (BAT) AGNs (23% ± 6%). The fraction of [O IV]-weak AGNs increases from ${27}_{-10}^{+13} \% $ (early) to ${66}_{-12}^{+10} \% $ (late mergers). Similar results are obtained with the [Ne V] line. The [O IV]- or [Ne V]-weak AGNs in late mergers show larger N _H and Eddington ratios (λ _Edd) than those of the Swift/BAT AGNs, and the largest N _H is ≳10²⁵ cm^‑2 at $\mathrm{log}{\lambda }_{\mathrm{Edd}}\sim -1$ , close to the effective Eddington limit for CT material. These suggest that (1) the circumnuclear material in buried AGNs is regulated by the radiation force from high-λ _Edd AGNs on the CT obscurers, and (2) their dense material with large ${f}_{\mathrm{CT}}^{(\mathrm{spec})}$ (∼0.5 ± 0.1) in U/LIRGs is a likely cause of a unique structure of buried AGNs, whose amount of material may be maintained through merger-induced supply from their host galaxies.