[O iv]- and [Ne v]-weak Active Galactic Nuclei 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 a [O iv ] 25.89 or [Ne v ] 14.32 μ m line to 12 μ m 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 <?CDATA fCT⁽ˢᵖᵉᶜ⁾=0.55± 0.19?> f CT ( spec ) = 0.55 ± 0.19 on average. This value is consistent with the fraction of CT AGNs ( <?CDATA fCT⁽ˢᵗᵃᵗ⁾=53 % ± 12 %?> f CT ( stat ) = 53 % ± 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 <?CDATA 27₋₁₀⁺¹³ %?> 27 − 10 + 13 % (early) to <?CDATA 66₋₁₂⁺¹⁰ %?> 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 ≳10 25 cm −2 at <?CDATA logλ Edd~ -1?> log λ 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 <?CDATA fCT⁽ˢᵖᵉᶜ⁾?> f CT ( 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.