Merging Galaxy Cluster Abell 2255 in Mid-Infrared
  We present the mid-infrared (MIR) observation of a nearby galaxy cluster,
Abell 2255 by the AKARI space telescope. Using the AKARI's continuous
wavelength coverage between 3-24 micron and the wide field of view, we
investigate the properties of cluster member galaxies to see how the infall of
the galaxies, the cluster substructures, and the cluster-cluster merger
influence their evolution. We show that the excess of MIR (11 micron) flux is a
good indicator to discriminate galaxies at different evolutionary stages, and
divide galaxies into three classes accordingly : strong MIR-excess (N3-S11>0.2)
galaxies that include both unobscured and obscured star-forming galaxies, weak
MIR-excess (-2.0<N3-S11<-1.2) galaxies that are quiescent, old (>5 Gyr)
galaxies where the MIR emission arises mainly from the circumstellar dust
around AGB stars, and intermediate MIR-excess (-1.2<N3-S11<0.2) galaxies in
between the two classes that are less than a few Gyrs old past the prime star
formation activity. With the MIR-excess diagnostics, we investigate how local
and cluster-scale environments affect the individual galaxies. We derive the
total star formation rate of ~130 Msun/yr for A2255 using the strong MIR-excess
galaxies, which is consistent with other clusters of galaxies at similar
redshifts and with similar masses. We find no strong evidence that supports
enhanced star formation neither inside the cluster nor in the substructure
region. The intermediate MIR-excess galaxies, representing galaxies in
transition from star-forming galaxies to quiescent galaxies, are located
preferentially at the medium density region or cluster substructures. Our
findings suggest that galaxies are being transformed from star-forming galaxies
into red, quiescent galaxies from the infall region through near the core,
which can be well-explained by the ram-pressure stripping as previous
simulation suggests.
