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| Paper: |
Galaxies Die in Groups: An IRAC Autopsy |
| Volume: |
399, Panoramic Views of Galaxy Formation and Evolution |
| Page: |
340 |
| Authors: |
Wilman, D.J.; Pierini, D.; Tyler, K.; McGee, S.L.; Oemler, A. Jr; Morris, S.L.; Balogh, M.L.; Bower, R.G.; Mulchaey, J.S. |
| Abstract: |
The most massive galaxies in the Universe are also the oldest. To overturn this apparent contradiction with hierarchical growth models, we focus on the group-scale haloes which host most of these galaxies. Our z ∼ 0.4 group sample is selected in redshift space from the CNOC2 redshift survey. A stellar mass selected M* ≥ 2 × 1010M⊙ sample is constructed using IRAC observations. A sensitive Mid InfraRed (MIR) IRAC colour is used to isolate passive galaxies. It produces a bimodal distribution, in which passive galaxies (highlighted by morphological early-types) define a tight MIR colour sequence (Infrared Passive Sequence, IPS). This is due to stellar atmospheric emission from old stellar populations. Significantly offset from the IPS are galaxies where reemission by dust boosts emission at λobs=8μm. We term them InfraRed-Excess galaxies whether star formation and/or AGN activity are present. They include all known morphological late-types. The fraction of InfraRed Excess galaxies, f(IRE) drops with M*, such that f(IRE) = 0.5 at a “crossover mass” of Mcr ∼ 1.3 × 1011M⊙. Within our optically-defined group sample there is a strong and consistent deficit in f(IRE) at all masses, but most clearly at M* ≥ 1011M⊙. Suppression of star formation must mainly occur in groups, and the observed trend of f(IRE) with M* can be explained if suppression of M* ≥ 1011M⊙ galaxies occurs primarily in the group environment. |
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