Luminosity Ratio between [O IV] 25.89 um Line and Nuclear Continuum 12
  um as Diagnostic for "Buried" AGN
  We propose new diagnostics that utilize the [O IV] 25.89 $\mu$m and nuclear
(subarcsecond scale) 12 $\mu$m luminosity ratio for identifying whether an AGN
is deeply `buried' in their surrounding material. 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 $\mu$m 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
ultra/luminous infrared galaxies (U/LIRGs) in various merger stages hosting
AGNs. We find that AGNs in most of mid- to late-stage mergers are buried, while
those in earlier stage ones (including non-merger) are not. This result
suggests that the fraction of buried AGNs in U/LIRGs increases as the
galaxy-galaxy interaction becomes more significant.
