ACCESS IV: The quenching of star formation in a cluster population of
  dusty S0s
  We present an analysis of the mid-infrared (MIR) colours of 165 70um-detected
galaxies in the Shapley supercluster core (SSC) at z=0.048 using panoramic
Spitzer/MIPS 24 and 70um imaging. While the bulk of galaxies show f70/f24
colours typical of local star-forming galaxies, we identify a significant
sub-population of 23 70micron-excess galaxies, whose MIR colours (f70/f24>25)
are much redder and cannot be reproduced by any of the standard model infrared
SEDs. These galaxies are found to be strongly concentrated towards the cores of
the five clusters that make up the SSC, and also appear rare among local field
galaxies, confirming them as a cluster-specific phenomenon. Their optical
spectra and lack of significant UV emission imply little or no ongoing star
formation, while fits to their panchromatic SEDs require the far-IR emission to
come mostly from a diffuse dust component heated by the general interstellar
radiation field rather than ongoing star formation. Most of these
70micron-excess galaxies are identified as ~L* S0s with smooth profiles. We
find that almost every cluster galaxy in the process of star-formation
quenching is already either an S0 or Sa, while we find no passive galaxies of
class Sb or later. Hence the formation of passive early-type galaxies in
cluster cores must involve the prior morphological transformation of late-type
spirals into Sa/S0s, perhaps via pre-processing or the impact of cluster tidal
fields, before a subsequent quenching of star formation once the lenticular
encounters the dense environment of the cluster core. In the cases of many
cluster S0s, this phase of star-formation quenching is characterised by an
excess of 70um emission, indicating that the cold dust content is declining at
a slower rate than star formation.
