ACCESS III: The Nature of Star Formation in the Shapley Supercluster
  We present a joint analysis of panoramic Spitzer/MIPS mid-infrared and GALEX
ultraviolet imaging of the Shapley supercluster at z=0.048. Combining this with
spectra of 814 supercluster members and 1.4GHz radio continuum maps, this
represents the largest complete census of star-formation (both obscured and
unobscured) in local cluster galaxies to date, reaching SFRs~0.02Msun/yr. We
take advantage of this comprehensive panchromatic dataset to perform a detailed
analysis of the nature of star formation in cluster galaxies, using several
quite independent diagnostics of the quantity and intensity of star formation
to develop a coherent view of the types of star formation within cluster
galaxies. We observe a robust bimodality in the infrared (f_24/f_K) galaxy
colours, which we are able to identify as another manifestation of the broad
split into star-forming spiral and passive elliptical galaxy populations seen
in UV-optical surveys. This diagnostic also allows the identification of
galaxies in the process of having their star formation quenched as the infrared
analogue to the UV "green valley" population. The bulk of supercluster galaxies
on the star-forming sequence have specific-SFRs consistent with local field
specific-SFR-M* relations, and form a tight FIR-radio correlation confirming
that their FIR emission is due to star formation. We show that 85% of the
global SFR is quiescent star formation within spiral disks, as manifest by the
observed sequence in the IRX-beta relation being significantly offset from the
starburst relation of Kong et al. (2004), while their FIR-radio colours
indicate dust heated by low-intensity star formation. Just 15% of the global
SFR is due to nuclear starbursts. The vast majority of star formation seen in
cluster galaxies comes from normal infalling spirals who have yet to be
affected by the cluster environment.
