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

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 [O IV] 25.89-um or [Ne V] 14.32-um line to 12-um 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≥ 10^24 cm^-2) material with an X-ray clumpy torus model to be f^ (spec)_ CT = 0.55±0.19 on average. This value is consistent with the fraction of CT AGNs (f^ (stat)_ CT = 53±12 larger than that in Swift/BAT AGNs (23±6 AGNs increases from 27^+13_-10 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 ≳10^25 cm^-2 at logλ_ Edd∼ -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^ (spec)_ CT (∼ 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.