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Astron. Astrophys. 343, 389-398 (1999)

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1. Introduction

Very little is known about the mid-IR (5 [FORMULA] 40µm) properties of distant galaxies, wavelengths which should probe their dust content and star formation activity. IRAS data established that mid-IR emission from the interstellar medium and from nearby star forming galaxies is dominated by emission from Very Small grains and polycyclic aromatic hydrocarbon molecules (PAHs) which have fluctuating temperatures under single photon absorption, whereas classical grains are in thermal equilibrium and emit at longer wavelengths (Helou 1986). However, the precise nature of PAHs remains open (Puget & Léger 1989). Differences in mid-IR features among various galaxies can be attributed to differing amounts of dust, broad emission interpreted as due to PAHs or Unidentified Infrared Bands (UIBs) carriers. The PAH and very small grains appear to be responsible for [FORMULA]30% of the total IR emission in normal galaxies, but in active galactic nuclei, the mid-IR emission is believed to arise from a dusty torus around the active nucleus where PAHs are destroyed (Edelson & Malkan 1986, Roche et al. 1991, Helou et al., 1991).

In order to study the mid-IR emission from high redshift galaxies, deep ISO (Infrared Space Observatory, Kessler et al. 1996) observations were made of the CFRS field at 1415+52. Extensive spectroscopic and [FORMULA] and K photometric data already exist (Lilly et al. 1995a,b) for galaxies in this field, as well as data from a deep ([FORMULA] µJy) radio survey by Fomalont et al. (1991). The high spatial resolution in the micro-scanning mode, combined with the good sensitivity of CAM (Césarsky et al. 1996) allows mid-IR maps of high redshift field galaxies to be made for the first time. Even so, precise identification of faint ISO sources with such galaxies is difficult, owing to their faintness and to the inherent uncertainties in the source positions.

The layout of this paper is as follows: Sect. 2 presents the observational and data reduction strategy; Sect. 3 gives a description of the catalogue, astrometry and identification of optical counterparts. In Sect. 4 the photometric and spectrophotometric properties of the ISO objects are discussed. Sect. 5 summarizes the results of the identification procedure and, finally, our conclusions about the faint mid-IR sources are discussed in Sect. 6.

The nature of 6.75µm sources and their energy distribution from UV to radio wavelengths will be fully discussed in a forthcoming paper (Flores et al. 1998), which will also present the 15µm data.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999