2000A&A...361..827F

We present the results of infrared observations of Abell 1689, an exceptionally rich cluster of galaxies at intermediate redshift (z≃0.181). It was observed with ISOCAM, at 6.7µmand 15µm, and ISOPHOT at 200µmfrom the Infrared Space Observatory (ISO). The cluster galaxies detected above a sensitivity limit of 0.15 mJy in the 6.7µmband, whose emission is mostly dominated by their stellar component, show optical colors similar to the overall cluster population and are gathered in the center of the cluster, following the distribution of the cluster early-types. In the 15µmband, above a sensitivity limit of 0.3 mJy, the galaxies spectroscopically confirmed to be cluster members are blue outliers of the cluster color-magnitude relation and become brighter going from the center to the outer parts of the cluster. We compare the 6.7µmand 15µmfluxes and the cumulative distributions of the B-[6.75] and B-[15] colors of the A1689 galaxies, above our 90% completeness limits of 0.2 and 0.4 mJy for 6.7µmand 15µmrespectively, to the galaxies of two nearby clusters, Virgo and Coma, and to the field galaxies at the same redshift of the cluster. Although the B-[6.7] color distributions of the three clusters are compatible, we find a systematic excess of B-[15] color distribution for the galaxies located in Abell 1689 with respect to Coma or Virgo galaxies. This result proves the existence of a mid-infrared equivalent of the Butcher-Oemler effect measured in the optical. The comparison of 15µmflux and B-[15] color distributions of A1689 and field galaxies does not show strong differences between the population of starburst galaxies in the cluster and in the field.