A Panoramic Mid‐Infrared Survey of Two Distant Clusters

(abridged) We present panoramic Spitzer MIPS 24um observations covering9x9Mpc (25'x25') fields around two massive clusters, Cl0024+16 and MS0451-03,at z=0.39 and z=0.55. Our observations cover a very wide range of environmentswithin these clusters, from high-density regions around the cores out to theturn-around radius. Cross-correlating the mid-infrared catalogs with deepoptical and near-infrared imaging of these fields, we investigate theoptical/near-infrared colors of the mid-infrared sources. We find excesses ofmid-infrared sources with optical/near-infrared colors expected of clustermembers in the two clusters and test this selection using spectroscopicallyconfirmed 24um members. The much more significant excess is associated withCl0024+16, whereas MS0451-03 has comparatively few mid-infrared sources. Themid-infrared galaxy population in Cl0024+16 appears to be associated with dustystar-forming galaxies (typically redder than the general cluster population byup to A_V~1-2 mags) rather than emission from dusty tori around active galacticnuclei (AGN) in early-type hosts. The inferred total-infrared star-formationrates in Cl0024+16 are typically >5x greater than those found from a similarHalpha survey, indicating significant obscured activity in the clusterpopulation. We find evidence for strong evolution of the level of dust-obscuredstar-formation in dense environments out to z=0.5, analogous to the rise infraction of optically-selected star-forming galaxies seen in clusters and thefield out to similar redshifts. However, there are clearly significantcluster-to-cluster variations in the populations of mid-infrared sources,probably reflecting differences in the intracluster media and recent dynamicalevolution of these systems.Comment: 13 pages, 6 figures, accepted for publication in Ap