A Panoramic Mid-infrared Survey of Two Distant Clusters

We present panoramic Spitzer MIPS 24-μm observations covering ∼9×9 Mpc (25 × 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-σ flux limit of ∼ 200μJy. Our observations cover a very wide range of environments within these clusters, from high-density regions around the cores out to the turn-around 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 optical/near-infrared colors expected of cluster members in the two clusters and test this selection using spectroscopically confirmed 24μm 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 AV ∼ 1–2 mags) rather than emission from dusty tori around active galactic nuclei (AGN) in early-type hosts. We compare the star-formation rates derived from the total infrared (8–1000μm) luminosities for the mid-infrared sources in Cl 0024+16 with those estimated from a published Hα survey, finding rates > ∼ 5× than those found from Hα, indicating significant obscured activity in the cluster population. Compared to previous mid-infrared surveys of clusters from z ∼ 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 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. Subject headings: galaxies: clusters – galaxies: starburst – galaxies: infrared – clusters: individual Cl 0024+16, MS 0451−03