The Mid-infrared View of Red Sequence Galaxies in Abell 2218 with AKARI
  We present the {\it AKARI} InfraRed Camera (IRC) imaging observation of
early-type galaxies in A2218 at z $\simeq$ 0.175. Mid-infrared (MIR) emission
from early-type galaxies traces circumstellar dust emission from AGB stars
or/and residual star formation. Including the unique imaging capability at 11
and 15 $\mu$m, our {\it AKARI} data provide an effective way to investigate MIR
properties of early-type galaxies in the cluster environment. Among our
flux-limited sample of 22 red sequence early-type galaxies with precise
dynamical and line strength measurements ($<$ 18 mag at 3 $\mu m$), we find
that at least 41% have MIR-excess emission. The $N3-S11$ versus $N3$ (3 and 11
$\mu$m) color-magnitude relation shows the expected blue sequence, but the
MIR-excess galaxies add a red wing to the relation especially at the fainter
end. A SED analysis reveals that the dust emission from AGB stars is the most
likely cause for the MIR-excess, with low level of star formation being the
next possible explanation. The MIR-excess galaxies show a wide spread of
$N3-S11$ colors, implying a significant spread (2--11 Gyr) in the estimated
mean ages of stellar populations. We study the environmental dependence of
MIR-excess early-type galaxies over an area out to a half virial radius
($\sim$1 Mpc). We find that the MIR-excess early-type galaxies are
preferentially located in the outer region. From these evidences, we suggest
that the fainter, MIR-excess early-type galaxies have just joined the red
sequence, possibly due to the infall and subsequent morphological/spectral
transformation induced by the cluster environment.
