AKARI Observation of the North Ecliptic Pole (NEP) Supercluster at z =
  0.087: mid-infrared view of transition galaxies
  We present the mid-infrared (MIR) properties of galaxies within a
supercluster in the North Ecliptic Pole region at z?0.087 observed with the
AKARI satellite. We use data from the AKARI NEP-Wide (5.4 deg2) IR survey and
the CLusters of galaxies EVoLution studies (CLEVL) mission program. We show
that near-IR (3 {\mu}m)-mid- IR (11 {\mu}m) color can be used as an indicator
of the specific star formation rate and the presence of intermediate age
stellar populations. From the MIR observations, we find that red-sequence
galaxies consist not only of passively evolving red early-type galaxies, but
also of 1) "weak-SFG" (disk-dominated star-forming galaxies which have star
formation rates lower by \sim 4 \times than blue-cloud galaxies), and 2)
"intermediate- MXG" (bulge-dominated galaxies showing stronger MIR dust
emission than normal red early-type galaxies). Those two populations can be a
set of transition galaxies from blue, star-forming, late-type galaxies evolving
into red, quiescent, early-type ones. We find that the weak-SFG are predominant
at intermediate masses (1010M\odot < M\star < 1010.5M\odot) and are typically
found in local densities similar to the outskirts of galaxy clusters. As much
as 40% of the supercluster member galaxies in this mass range can be classified
as weak-SFGs, but their proportion decreases to < 10% at larger masses (M\star
> 1010.5 M\odot) at any galaxy density. The fraction of the intermediate-MXG
among red- sequence galaxies at 1010M\odot < M\star < 1011M\odot also decreases
as the density and mass increase. In particular, \sim42% of the red-sequence
galaxies with early-type morphologies are classified as intermediate-MXG at
intermediate densities. These results suggest that the star formation activity
is strongly dependent on the stellar mass, but that the morphological
transformation is mainly controlled by the environment.
