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Astron. Astrophys. 343, L65-L69 (1999)

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Letter to the Editor

An ultra-deep ISOCAM observation through a cluster-lens *

B. Altieri 1, L. Metcalfe 1, J.P. Kneib 2, B. McBreen 3, H. Aussel 4, A. Biviano 5, M. Delaney 3, D. Elbaz 4, K. Leech 1, L. Lémonon 4, K. Okumura 6, R. Pelló 2 and B. Schulz 1

1 ISO Data Centre, Astrophysics Division, Space Science Department of ESA, Villafranca del Castillo, P.O. Box 50727, E-28080 Madrid, Spain
2 Observatoire Midi-Pyréneés, 14 Av. E. Belin, F-31400 Toulouse, France
3 Physics Department, University College Dublin, Stillorgan Rd., Dublin 4, Ireland
4 DSM/DAPNIA/SAp, CEA-Saclay, F-91191 Gif-sur-Yvette Cedex, France
5 Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, I-34131 Trieste, Italy
6 Institut d'Astrophysique Spatiale, Bât. 121, Université Paris-Sud, F-91405 Orsay, France

Received 30 October 1998 / Accepted 3 February 1999


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 7 µm sources are cluster galaxies. The other 7[FORMULA]m 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 [FORMULA] and are large, with [FORMULA] 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[FORMULA]0.5)[FORMULA]10-9 and (3.3[FORMULA]1.3)[FORMULA]10-9 W m-2 sr-1.

Key words: galaxies: abundances – galaxies: clusters: general – galaxies: evolution – cosmology: gravitational lensing – infrared: galaxies

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA

Send offprint requests to: B. Altieri (baltieri@iso.vilspa.esa.es)

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999