The dark side of star formation in galaxy clusters: spectroscopic follow-up of clusters observed with ISOCAM

Abstract

The evolution of galaxies in cluster environments can be studied using mid-IR observations which trace star forming regions hidden by dust. The optical follow-up of A1689 (z=0.2), observed at 6.7 and 15um by ISOCAM, have revealed a systematic excess of the B-[15um] galaxy color distribution with respect that of Coma and Virgo clusters. This result suggests the existence of a dark side of the Butcher-Oemler effect measured in the optical. We present an analysis of the optical/mid-IR properties of the mid-IR emitters in A1689, comparing in particular the star formation rates based on mid-IR and optical data. Morover, we present preliminary result for J1888, a cluster at z=0.56 deeply observed with ISOCAM, based on recent VLT/FORS and NTT/SOFI observations.