# Luminosity Ratio between [O IV] 25.89 um Line and Nuclear Continuum 12 um as Diagnostic for "Buried" AGN

## TL;DR

This paper introduces a new diagnostic method using the luminosity ratio of [O IV] 25.89 μm to 12 μm to identify deeply buried AGNs, revealing that buried AGNs are more common in advanced galaxy mergers.

## Contribution

The study proposes a novel luminosity ratio diagnostic for buried AGNs and demonstrates its effectiveness across a sample of absorbed AGNs and U/LIRGs at various merger stages.

## Key findings

- Buried AGNs show lower [O IV]-to-12 μm luminosity ratios.
- Most mid- to late-stage mergers host buried AGNs.
- The fraction of buried AGNs increases with merger progression.

## Abstract

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.