Leo Metcalfe 1, B. Altieri 1, A. Biviano 2, B. McBreen 3, M. Delaney 3, D. Elbaz 4, M.F. Kessler 1, J-P. Kneib 5, K. Leech 1, K. Okumura 5, & B. Schulz 1
1 ISO Science Operations Centre, Astrophysics Division, Space Science Department of ESA, Villafranca, P.O. Box 50727, 28080 Madrid, Spain.
2 Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 1-34131 Trieste, Italy.
3 Physics Department, University College Dublin, Stillorgan Road, Dublin 4, Ireland.
4 CEA/Service d'Astrophysique, C.E. Saclay, F-91191 Gif-sur-Yvette, France.
5 Observatoire Midi-Pyrenees, 14 Av. E.Belin, 31400 Toulouse, France.
6 Institut d'Astrophysique Spatiale, University Paris XI, Bat.121, F-91405, Orsay, France.
We present the results of an ISOCAM deep and ultra-deep cosmological survey in which 40 hours of ISO time were used to survey, through gravitationally lensing galaxy clusters, about 70 square arc minutes of sky, using CAM's LW2 (6.7 micron) and LW3 (14.3 micron) filters. The three sigma sensitivity limits in LW2 range from 60 microJy down to 20 microJy, and the LW3 limits range from 40 microJy down to 130 microJy, the limits depending on the lensing cluster in question.
Apart from the usage of cluster lenses to increase the depth of the survey, a relatively unique feature of this programme has been its usage of the CAM 3" per pixel field-of-view, giving significantly better spatial resolution than achieved in most other CAM deep surveys, which relied on the 6" PFOV.
Results are presented for imaging through the clusters Abell 370, Abell 2218, and Abell 2390. We present 7 and 15 micron images of each cluster covering areas of 36, 24 and 9 square arc-minutes respectively. These cluster images, unprecedented in themselves for their depth and coverage at these wavelengths, reveal a large number of luminous mid-infrared (MIR) sources behind the lensing clusters. Cross-identification with optical and near-infrared (NIR) images shows that about half of the 7 micron sources are cluster galaxies, yet the other 7 micron, and almost all of the 15 micron, sources are identified as lensed, distant, galaxies.
Thanks to the gravitational amplification of the lensing clusters, a significant subset of these detections constitute the faintest MIR detections ever recorded, allowing us to extend source counts to an unprecedented level. In deep field surveys, absolute astrometry determination of MIR sources is difficult because of the lack of obvious NIR/optical counterparts. In the data presented here, unambiguous identification of more than 90% of the sources was possible, thanks to a large density of sources and good sampling of the PSF.
These resulting source counts, corrected for cluster contamination and lensing distortion effects, show an excess by a factor of 10 with respect to the prediction of a no-evolution model.