AKARI OBSERVATION OF THE NORTH ECLIPTIC POLE (NEP) SUPERCLUSTER AT z=0.087: MID-INFRARED VIEW OF TRANSITION GALAXIES
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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 (5.4deg2) IR survey and the CLusters of galaxies EVoLution studies (CLEVL) mission program. We show that near-IR (3 m)-mid-IR (11 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 4 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 (1010 M < M * < 1010.5 M ) 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 ) at any galaxy density. The fraction of the intermediate-MXG among red-sequence galaxies at 1010 M < M * < 1011 M also decreases as the density and mass increase. In particular, 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. 2012. The American Astronomical Society. All rights reserved.
