Unveiling the Important Role of Groups in the Evolution of Massive Galaxies: Insights from an Infrared Passive Sequence at Intermediate Redshift

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 halos that 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 × 1010 M☉ sample is constructed using IRAC observations. A sensitive mid-infrared (MIR) IRAC color is used to isolate passive galaxies. It produces a bimodal distribution, in which passive galaxies (highlighted by morphological early types) define a tight MIR color 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 = 8 μm. We term them infrared excess galaxies, whether star formation and/or AGN activity are present. They include all known morphological late types. Comparison with EW[O II] shows that MIR color is highly sensitive to low levels of activity and allows us to separate dusty active from passive galaxies at high stellar mass. 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 × 1011 M☉. Within our optically defined group sample there is a strong and consistent deficit in f(IRE) at all masses, but most clearly at M*≳ 1011 M☉. Suppression of star formation must mainly occur in groups. In particular, the observed trend of f(IRE) with M* can be explained if suppression of M*≳ 1011 M☉ galaxies occurs primarily in the group environment. This is confirmed using a mock galaxy catalog derived from the millenium simulation. In this way, the mass-dependent evolution in f(IRE) (downsizing) can be driven solely by structure growth in the universe, as more galaxies are accreted into group-sized halos with cosmic time.