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

Abstract

We present the results of infrared observations of Abell 1689, an exceptionally rich cluster of galaxies at intermediate redshift (z^simeq 0.181). It was observed with ISOCAM, at 6.7 mum and 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 mum band, 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 15 mum band, 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 6.7 mum and 15 mum fluxes 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 6.7 mum and 15 mum respectively, 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 15 mum flux 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA