[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]/L12,AGN ≤ -3.0) or [Ne V]-weak (L[Ne\,V]/L12,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; NH ≥ 10²⁴ 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%) among the [O IV]-weak AGNs in U/LIRGs and much larger than that in Swift/BAT AGNs (23±6%). The fraction of [O IV]-weak AGNs increases from 27⁺¹³₋₁₀% (early) to 66⁺¹⁰₋₁₂% (late mergers). Similar results are obtained with the [Ne V] line. The [O IV] or [Ne V]-weak AGNs in late mergers show larger NH and Eddington ratios (λEdd) than those of the Swift/BAT AGNs, and the largest NH is 10²⁵ cm⁻² at logλEdd ~ -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-λEdd 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.