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The most massive galaxies in the Universe are also the oldest. To overturnthis apparent contradiction with hierarchical growth models, we focus on thegroup scale haloes which host most of these galaxies. A stellar mass selectedM_* >~ 2x10^10M_sol sample at z~0.4 is constructed within the CNOC2 redshiftsurvey. A sensitive Mid InfraRed (MIR) IRAC colour is used to isolate passivegalaxies. 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 emissionfrom old stellar populations. Significantly offset from the IPS are galaxieswhere reemission by dust boosts emission at 8microns (InfraRed-Excess or IREgalaxies). They include all known morphological late-types. Comparison withEW[OII] shows that MIR colour is highly sensitive to low levels of activity,and allows us to separate dusty-active from passive galaxies. The fraction ofIRE galaxies, f(IRE) drops with M_*, such that f(IRE)=0.5 at a ``crossovermass'' of ~1.3x10^11M_sol. Within our optically-defined group sample there is astrong and consistent deficit in f(IRE) at all masses, and most clearly at M_*>~10^11M_sol. Using a mock galaxy catalogue derived from the MilleniumSimulation we show that the observed trend of f(IRE) with M_* can be explainedif suppression of star formation occurs primarily in the group environment, andparticularly for M_*>~10^11M_sol galaxies. In this way, downsizing can bedriven solely by structure growth in the Universe.

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