  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. A stellar mass selected
M_* >~ 2x10^10M_sol sample at z~0.4 is constructed within the CNOC2 redshift
survey. 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 8microns (InfraRed-Excess or IRE
galaxies). They include all known morphological late-types. Comparison with
EW[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 of
IRE galaxies, f(IRE) drops with M_*, such that f(IRE)=0.5 at a ``crossover
mass'' of ~1.3x10^11M_sol. Within our optically-defined group sample there is a
strong 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 Millenium
Simulation we show that the observed trend of f(IRE) with M_* can be explained
if suppression of star formation occurs primarily in the group environment, and
particularly for M_*>~10^11M_sol galaxies. In this way, downsizing can be
driven solely by structure growth in the Universe.

Astrophysics