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AKARI observation of the north ecliptic pole (NEP) supercluster at z = 0.087: Mid-infrared view of transition galaxies

  • Author(s): Ko, J
  • Im, M
  • Lee, HM
  • Lee, MG
  • Kim, SJ
  • Shim, H
  • Jeon, Y
  • Hwang, HS
  • Willmer, CNA
  • Malkan, MA
  • Papovich, C
  • Weiner, BJ
  • Matsuhara, H
  • Oyabu, S
  • Takagi, T
  • et al.

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 μm)-mid-IR (11 μm) 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.

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