A panoramic mid-infrared survey of two distant clusters

We present panoramic Spitzer MIPS 24 mu m observations, covering similar to 9 x 9 Mpc(2) (25' x 25') fields around two massive clusters, Cl 0024+16 and MS 0451-03, at z = 0.39 and z = 0.55, respectively, reaching a 5 sigma flux limit of similar to 200 mu Jy. Our observations cover a very wide range of environments within these clusters, from high-density regions around the cores out to the turnaround radius. Cross-correlating the mid-infrared catalogs with deep optical and near-infrared imaging of these fields, we investigate the optical/near-infrared colors of the mid-infrared sources. We find excesses of mid-infrared sources with the optical/near-infrared colors expected of cluster members in the two clusters and test this selection using spectroscopically confirmed 24 mu m members. The much more significant excess is associated with Cl 0024 + 16, whereas MS 0451 - 03 has comparatively few mid-infrared sources. The mid-infrared galaxy population in Cl 0024+16 appears to be associated with dusty star- forming galaxies (typically redder than the general cluster population by up to A(V) similar to 1-2 mag) rather than emission from dusty tori around active galactic nuclei in early-type hosts. We compare the star formation rates derived from the total infrared (8-1000 mu m) luminosities for the mid-infrared sources in Cl 0024 + 16 with those estimated from a published H alpha survey, finding rates greater than or similar to 5 times those found from H alpha, indicating significant obscured activity in the cluster population. Compared to previous mid-infrared surveys of clusters from z similar to 0-0.5, we find evidence for strong evolution of the level of dust-obscured star formation in dense environments to z 0: 5, analogous to the rise in the fraction of optically selected star- forming galaxies seen in clusters and the field out to similar redshifts. However, there are clearly significant cluster-to-cluster variations in the populations of mid-infrared sources, probably reflecting differences in the intracluster media and recent dynamical evolution of these systems.