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 similar to 0.087 observed with the AKARI satellite. We use data from the AKARI NEP-Wide (5.4 deg(2)) 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-SFGs (disk-dominated star-forming galaxies that have star formation rates lower by similar to 4 x than blue-cloud galaxies) and (2) intermediate-MXGs (bulge-dominated galaxies showing stronger MIR dust emission than normal red early-type galaxies). These 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-SFGs are predominant at intermediate masses (10(10) M-circle dot < M-* < 10(10.5) M-circle dot) 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-* > 10(10.5) M-circle dot) at any galaxy density. The fraction of the intermediate-MXG among red-sequence galaxies at 10(10) M-circle dot < M-* < 10(11) M-circle dot also decreases as the density and mass increase. In particular, similar to 42% of the red-sequence galaxies with early-type morphologies are classified as intermediate-MXGs 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.