Environmental Effects on AGN activity via Extinction-free Mid-Infrared
  Census
  How does the environment affect active galactic nucleus (AGN) activity? We
investigated this question in an extinction-free way, by selecting 1120
infrared galaxies in the $AKARI$ North Ecliptic Pole Wide field at redshift $z$
$\leq$ 1.2. A unique feature of the $AKARI$ satellite is its continuous 9-band
infrared (IR) filter coverage, providing us with an unprecedentedly large
sample of IR spectral energy distributions (SEDs) of galaxies. By taking
advantage of this, for the first time, we explored the AGN activity derived
from SED modelling as a function of redshift, luminosity, and environment. We
quantified AGN activity in two ways: AGN contribution fraction (ratio of AGN
luminosity to the total IR luminosity), and AGN number fraction (ratio of
number of AGNs to the total galaxy sample). We found that galaxy environment
(normalised local density) does not greatly affect either definitions of AGN
activity of our IRG/LIRG samples (log ${\rm L}_{\rm TIR}$ $\leq$ 12). However,
we found a different behavior for ULIRGs (log ${\rm L}_{\rm TIR}$ $>$ 12). At
our highest redshift bin (0.7 $\lesssim$ z $\lesssim$ 1.2), AGN activity
increases with denser environments, but at the intermediate redshift bin (0.3
$\lesssim$ z $\lesssim$ 0.7), the opposite is observed. These results may hint
at a different physical mechanism for ULIRGs. The trends are not statistically
significant (p $\geq$ 0.060 at the intermediate redshift bin, and p $\geq$
0.139 at the highest redshift bin). Possible different behavior of ULIRGs is a
key direction to explore further with future space missions (e.g., $JWST$,
$Euclid$, $SPHEREx$).
