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Ko, Jongwan,Im, Myungshin,Lee, Hyung Mok,Lee, Myung Gyoon,Kim, Seong Jin,Shim, Hyunjin,Jeon, Yiseul,Hwang, Ho Seong,Willmer, Christopher N. A.,Malkan, Matthew A.,Papovich, Casey,Weiner, Benjamin J.,Ma IOP Publishing 2012 The Astrophysical journal Vol.745 No.2
<P>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.</P>
THE MID-INFRARED AND NEAR-ULTRAVIOLET EXCESS EMISSIONS OF QUIESCENT GALAXIES ON THE RED SEQUENCE
Ko, Jongwan,Hwang, Ho Seong,Lee, Jong Chul,Sohn, Young-Jong IOP Publishing 2013 The Astrophysical journal Vol.767 No.1
<P>We study the mid-infrared (IR) and near-ultraviolet (UV) excess emissions of spectroscopically selected quiescent galaxies on the optical red sequence. We use the Wide-field Infrared Survey Explorer mid-IR and Galaxy Evolution Explorer near-UV data for a spectroscopic sample of galaxies in the Sloan Digital Sky Survey Data Release 7 to study the possible connection between quiescent red-sequence galaxies with and without mid-IR/near-UV excess. Among 648 12 mu m detected quiescent red-sequence galaxies without Ha emission, 26% and 55% show near-UV and mid-IR excess emissions, respectively. When we consider only bright (M-r < -21.5) galaxies with an early-type morphology, the fraction of galaxies with recent star formation is still 39%. The quiescent red-sequence galaxies with mid-IR and near-UV excess emissions are optically fainter and have slightly smaller D(n)4000 than those without mid-IR and near-UV excess emissions. We also find that mid-IR weighted mean stellar ages of quiescent red-sequence galaxies with mid-IR excess are larger than those with near-UV excess, and smaller than those without mid-IR and near-UV excess. The environmental dependence of the fraction of quiescent red-sequence galaxies with mid-IR and near-UV excess seems strong even though the trends of quiescent red-sequence galaxies with near-UV excess differ from those with mid-IR excess. These results indicate that the recent star formation traced by near-UV (less than or similar to 1 Gyr) and mid-IR (less than or similar to 2 Gyr) excess is not negligible among nearby, quiescent, red, early-type galaxies. We suggest a possible evolutionary scenario of quiescent red-sequence galaxies from quiescent red-sequence galaxies with near-UV excess to those with mid-IR excess to those without near-UV and mid-IR excess.</P>