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

[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 ($L_{rm [O,IV]}$/$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 $f^{rm (spec)}_{rm CT} = 0.55pm0.19$ on average. This value is consistent with the fraction of CT AGNs ($f^{rm (stat)}_{rm CT} = 53pm12$%) among the [O IV]-weak AGNs in U/LIRGs and much larger than that in Swift/BAT AGNs ($23pm6$%). 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.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 $f^{rm (spec)}_{rm CT} = 0.55pm0.19$ on average. This value is consistent with the fraction of CT AGNs ($f^{rm (stat)}_{rm CT} = 53pm12$%) among the [O IV]-weak AGNs in U/LIRGs and much larger than that in Swift/BAT AGNs ($23pm6$%). 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.