Letter to the Editor An ultra-deep ISOCAM observation through a cluster-lens

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. Crossidentification in the optical and Near-Infrared (NIR) wavebands shows that about half of the 7 μm sources are cluster galaxies. The other 7 μm and almost all 15 μm 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 15 μm bands. In particular, we find that the 15 μm counts have a steep slope α15μm = −1.5 ± 0.3 and are large, with N15μm(> 30μJy) = 13 ± 5 arcmin−2. These numbers rule out non-evolutionary models and favour very strong evolution. Down to our counts limit, we found that the resolved 7 μm and 15 μm background radiation intensities are respectively (1.7±0.5)×10−9 and (3.3±1.3)×10−9 W m−2 sr−1.