ISO's infrared camera was used to make deep mid-infrared (MIR) images through three gravitationally lensing clusters of galaxies. Observations were made at 7 μm and 15 μm covering more than 50 square arcminutes, with the lensing increasing the sensitivity to background sources significantly.
A large number of MIR sources were detected behind the lenses and provide source counts, corrected for cluster contamination and lensing distortion effects, which exceed by a factor of 10 the expectation from local counts assuming a no-evolution model. The results are consistent with larger-area surveys and the detected population resolves a substantial fraction (of order 60%) of the background MIR radiation intensity into discrete sources.
We discuss the evidence, in large part derived from lensing cluster observations, for overlap of the ISO 15 μm faint galaxy population with the 850 μm submillimetre and the 0.5 to 7 keV X-ray populations. We find that the ISO data shows substantial overlap with both the submillimetre and the X-ray source populations, with roughly 25% of ISO sources being detected at submillimetre wavelengths and a significant number of Chandra X-ray sources being detected in the ISO data.