1999A&A...343L..65A

We present results of ultra-deep ISOCAM observations through a cluster-lens at 7µm and 15µm with the Infrared Space Observatory (ISO) satellite. These observations reveal a large number of luminous Mid-Infrared (MIR) sources. Cross-identification in the optical and Near-Infrared (NIR) wavebands shows that about half of the 7µm sources are cluster galaxies. The other 7µm and almost all 15µm sources are identified as lensed distant galaxies. Thanks to the gravitational amplification they constitute the faintest sources detected in the MIR, allowing us to extend the number counts in both the 7 and 15µm bands. In particular, we find that the 15µm counts have a steep slope α_15µm=-1.5±0.3 and are large, with N_15µm(>30µJy)=13±5arcmin^–2. These numbers rule out non-evolutionary models and favour very strong evolution. Down to our counts limit, we found that the resolved 7µm and 15µm background radiation intensities are respectively (1.7±0.5)x10^–9 and (3.3±1.3)x10^–9W/m²/sr.