DUST-OBSCURED STAR FORMATION IN INTERMEDIATE REDSHIFT GALAXY CLUSTERS

Abstract

We present Spitzer MIPS 24-micron observations of 16 0.4<z<0.8 galaxy\nclusters drawn from the ESO Distant Cluster Survey (EDisCS). This is the first\nlarge 24-micron survey of clusters at intermediate redshift. The depth of our\nimaging corresponds to a total IR luminosity of 8x10^10 Lsun, just below the\nluminosity of luminous infrared galaxies (LIRGs), and 6^{+1}_{-1}% of M_V < -19\ncluster members show 24-micron emission at or above this level. We compare with\na large sample of coeval field galaxies and find that while the fraction of\ncluster LIRGs lies significantly below that of the field, the IR luminosities\nof the field and cluster galaxies are consistent. However, the stellar masses\nof the EDisCS LIRGs are systematically higher than those of the field LIRGs. A\ncomparison with optical data reveals that ~80% of cluster LIRGs are blue and\nthe remaining 20% lie on the red sequence. Of LIRGs with optical spectra,\n88^{+4}_{-5}% show [O II] emission with EW([O II])>5A, and ~75% exhibit optical\nsignatures of dusty starbursts. On average, the fraction of cluster LIRGs\nincreases with projected cluster-centric radius but remains systematically\nlower than the field fraction over the area probed (< 1.5xR200). The amount of\nobscured star formation declines significantly over the 2.4 Gyr interval\nspanned by the EDisCS sample, and the rate of decline is the same for the\ncluster and field populations. Our results are consistent with an exponentially\ndeclining LIRG fraction, with the decline in the field delayed by ~1 Gyr\nrelative to the clusters.\n