2004A&A...425...33B

We have observed the cluster Abell 2218 (z=0.175) with ISOCAM on board the Infrared Space Observatory using two filters, LW2 and LW3, with reference wavelengths of 6.7 and 14.3µm, respectively. We detected 76 sources down to 54 and 121µJy (50% completeness levels) at 6.7 and 14.3µm, respectively. All these sources have visible optical counterparts. We have gathered optical and near-infrared magnitudes for 60 of the 67 non-stellar optical counterparts to the ISOCAM sources, as well as redshifts for 43 of them. We have obtained acceptable and well constrained fits to the observed spectral energy distributions (SEDs) of 41 of these sources, using the ``GRASIL'' models of Silva et al. (1998ApJ...509..103S), and have determined their total infrared luminosities (L_IR's) and star formation rates (SFRs). The SEDs of 20 (out of 27) ISOCAM cluster members are best fit by models with negligible ongoing star formation, and no major episode of star formation in the last ∼1Gyr. Their SEDs resemble those of 5-10Gyr old early-type galaxies. A slightly higher, but still very mild, star-formation activity is found among the remaining cluster sources, which are mostly spirals. The median IR luminosity of the 27 ISOCAM cluster sources is L_IR=6x10⁸L_☉. The ISOCAM-selected cluster galaxies have indistinguishable velocity and spatial distributions from those of the other cluster galaxies, and do not contribute significantly to the Butcher-Oemler effect. If A2218 is undergoing a merger, as suggested by some optical and X-ray analyses, then this merger does not seem to affect the mid-infrared properties of its galaxies. The SEDs of most ISOCAM-selected field sources are best fit by models with moderate ongoing star formation, with a significant fraction of their stellar mass formed in the last ∼1 Gyr. Their SEDs resemble those of massive star-forming spirals or starburst galaxies, observed close to the maximum of their star formation activity, but not necessarily during the short-lived starburst event. The median redshift of these field galaxies is z≃0.6. Their L_IR's span almost two orders of magnitudes, from ∼10¹⁰L_☉ to ∼10¹²L_☉, with a median of 1.2x10¹¹ (eight of the 14 field sources are LIRGs). The SFRs of these 14 ISOCAM-selected field sources range from 2 to 125M_☉/yr, with a median value of 22M_☉/yr. We compare our findings with those obtained in other ISOCAM cluster and field surveys.