An excess of mid-IR luminous galaxies in Abell 1689?

We present the results of infrared observations of Abell 1689, an exception ally rich cluster of galaxies at intermediate redshift (z similar or equal to 0.181). It was observed with ISOCAM, at 6.7 mu mand 15 mum, and ISOPHOT at 200 mum from the Infrared Space Observatory (ISO). The cluster galaxies detected above a sensitivity limit of 0.15 mJy in the 6.7 mu mband, whose e mission is mostly dominated by their stellar component, show optical colors similar to the overall cluster population and are gathered in the center o f the cluster, following the distribution of the cluster early-types. In th e 15 mu mband, above a sensitivity limit of 0.3 mJy, the galaxies spectrosc opically confirmed to be cluster members are blue outliers of the cluster c olor-magnitude relation and become brighter going from the center to the ou ter parts of the cluster. We compare the 6.7 mu mand 15 mu mfluxes and the cumulative distributions o f the B-[6.75] and B-[15] colors of the A1689 galaxies, above our 90% compl eteness limits of 0.2 and 0.4 mJy for 6.7 mu mand 15 mum respectively, to t he galaxies of two nearby clusters, Virgo and Coma, and to the field galaxi es at the same redshift of the cluster. Although the B-[6.7] color distribu tions of the three clusters are compatible, we find a systematic excess of B-[15] color distribution for the galaxies located in Abell 1689 with respe ct to Coma or Vir,oo galaxies. This result proves the existence of a mid-in frared equivalent of the Butcher-Oemler effect measured in the optical. The comparison of 15 mu mflux and B-[15] color distributions of A1689 and fiel d galaxies does not show strong differences between the population of starb urst galaxies in the cluster and in the field.