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 ($L_{ \rm [O\,IV]}$/$L_{ \rm 12,AGN} \leq -$3.0) or [Ne V]-weak ($L_{ \rm [Ne\,V]}$/$L_{ \rm 12,AGN} \leq -$3.4) among 30 AGNs in local U/LIRGs. For the [O IV]-weak AGNs, we estimate their covering fractions of Compton-thick (CT; $N_{ \rm H} \geq 10^{ 24}$ cm$^{ -2}$) material with an X-ray clumpy torus model to be $f^{ \rm (spec)}_{ \rm CT} = 0.55\pm0.19$ on average. This value is consistent with the fraction of CT AGNs ($f^{ \rm (stat)}_{ \rm CT} = 53\pm12$%) among the [O IV]-weak AGNs in U/LIRGs and much larger than that in Swift/BAT AGNs ($23\pm6$%). 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 $N_{ \rm H}$ and Eddington ratios ($\lambda_{ \rm Edd}$) than those of the Swift/BAT AGNs, and the largest $N_{ \rm H}$ is $\gtrsim$10$^{ 25}$ cm$^{ -2}$ at ${ \log}\lambda_{ \rm Edd} \sim -$1, close to the effective Eddington limit for CT material. These suggest that (1) the circumnuclear material in buried AGNs is regulated by the radiation force from high-$\lambda_{ \rm Edd}$ AGNs on the CT obscurers, and (2) their dense material with large $f^{ \rm (spec)}_{ \rm CT}$ ($\sim 0.5 \pm 0.1$) 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.
Categories: astro-ph.GA astro-ph.HE