ACCESS - III. The nature of star formation in the Shapley supercluster

We present a joint analysis of panoramic Spitzer/MIPS mid- and far-infrared (MIR/FIR) and GALEX ultraviolet imaging of the most massive and dynamically active structure in the local Universe, the Shapley supercluster at z = 0.048, covering the five clusters that make up the supercluster core. Combining this with existing spectroscopic data from 814 confirmed supercluster members and 1.4-GHz radio continuum maps, this represents the largest complete census of star formation (both obscured and unobscured) in local cluster galaxies to date, extending down to star formation rates (SFRs) similar to 0.02-0.05 M-circle dot yr-1. We take advantage of this comprehensive panchromatic data set to perform a detailed analysis of the nature of star formation in cluster galaxies of the kind previously limited to local field galaxy surveys such as SINGS, 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 mu m)/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 ultraviolet 'green valley' population. The bulk of supercluster galaxies on the star-forming sequence have specific-SFRs consistent with local field specific-SFR-relations of Elbaz et al. (2007), and form a tight FIR-radio correlation (0.24 dex) confirming that their FIR emission is due to star formation. We show that 85 per cent of the global SFR is quiescent star formation within spiral discs, as manifest by the observed sequence in the (L-IR/F-FUV)-(FUV-NUV) plane - the IRX (infrared-to-ultraviolet ratio)-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 per cent of the global SFR is due to nuclear starbursts. The vast majority of star formation seen in cluster galaxies comes from normal infalling spirals that have yet to be affected by the cluster environment.