Luminosity Ratio between [O IV] 25.89 μm Line and Nuclear Continuum 12 μm as a Diagnostic for “Buried” AGNs

We propose new diagnostics that utilize the [O IV] 25.89 μm and nuclear (subarcsecond scale) 12 μm luminosity ratios for identifying whether an active galactic nucleus (AGN) is deeply “buried” in their surrounding materials. Utilizing a sample of 16 absorbed AGNs at redshifts lower than 0.03 in the Swift/BAT catalog observed with Suzaku, we find that AGNs with small scattering fractions (<0.5%) tend to show weaker [O IV]-to-12 μm luminosity ratios than the average of Seyfert 2 galaxies. This suggests that this ratio is a good indicator for identifying buried AGNs. Then, we apply this criterion to 23 local luminous and ultraluminous infrared galaxies (U/LIRGs) in various merger stages hosting AGNs. We find that AGNs in most mid- to late-stage mergers are buried, while those in earlier-stage ones (including nonmerger) are not. This result suggests that the fraction of buried AGNs in U/LIRGs increases as the galaxy-galaxy interaction becomes more significant.