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

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1. Introduction

Great progress in the understanding of physical properties of galaxies has been achieved with Mid-Infrared (MIR) and Far Infrared (FIR) observations using the ISO satellite (Kessler et al. 1996) and its ISOCAM camera (Cesarsky et al. 1996).

Deep optical surveys revealed a new population of distant sources at high-redshift ([FORMULA]), either using `drop-out' techniques (Steidel et al. 1996), or `photometric redshift' (e.g. Lanzetta et al. 1996). Yet optical surveys may miss a whole class of high-redshift dust-enshrouded galaxies (Blain & Longair 1993). Indeed, galaxy formation models (e.g. Arimoto & Yoshi 1987, Guiderdoni et al. 1997) predict that galaxies in their forming phase are heavily obscured by abundant gas and dust inside the system. These issues have motivated very deep ISOCAM observations of blank fields (Rowan-Robinson et al. 1997; Taniguchi et al. 1997), as well as Sub-millimeter observations (Smail et al. 1997; Barger et al. 1998; Hughes et al. 1998).

The ISOCAM-HDF observations showed that 15µm results rule out a no-evolution model (Oliver et al. 1997). Analysing galaxy counts over a wide range of sensitivity, Elbaz et al. (1998) noted a change of slope in the counts observed at the mJy-level. This variation requires stronger star-formation activity at moderate redshifts.

We report here very deep ISOCAM observations conducted through a gravitationally lensing cluster of galaxies as part of a large programme using gravitational lensing to achieve fainter detection thresholds in a given observation time. The full programme is reported elsewhere (Metcalfe et al. 1999a and b). By pushing ISOCAM to its ultimate limits with the help of gravitational lensing, we achieved ultra-deep observations through the core of probably the best studied lensing cluster: Abell 2390 ([FORMULA]), to obtain a magnified view of the background sky. This both increases the sensitivity of our MIR maps and reduces the effects of source confusion.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999