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Astron. Astrophys. 358, 30-44 (2000)

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1. Introduction

The measurement of weak gravitational lensing produced by the large-scale structures in the universe (hereafter, the cosmic shear) is potentially the most effective, albeit challenging, step toward a direct mapping of the dark matter distribution in the universe at intermediate and low redshift. Unlike several popular probes of large-scale structures, lensing maps the dark matter directly, regardless of the distribution of light emitted by gas and galaxies or the dynamical stage of the structures analysed.

A decade of theoretical and technical studies has shown that the gravitational distortion produced by the structures along the lines-of-sight contains important clues on structure formation models (see Mellier 1999, Bartelmann & Schneider 1999a for reviews and references therein). From these studies, we know that weak lensing can provide measurements of cosmological parameters and the shape of the projected density power spectrum (Blandford et al. 1991, Miralda-Escude 1991, Kaiser 1992, Villumsen 1996, Bernardeau et al. 1997, Jain & Seljak 1997, Kaiser 1998, Schneider et al. 1998, Jain et al. 1999, Van Waerbeke et al. 1999, Bartelmann & Schneider 1999b). However, it is also clear that the most challenging issues are observationals, because the measurement of extremely weak gravitational distortions is severely affected by various sources of noise and systematics such as the photon noise, the optical distortion of astronomical telescopes and the atmospheric distortion. Therefore, the problem of reliable shape measurement has also received much attention in the last few years (Bonnet & Mellier 1995, Kaiser et al. 1995, Van Waerbeke et al. 1997, Hoekstra et al. 1998, Kuijken 1999, Rhodes et al. 1999, Kaiser 1999, Bertin 2000).

Despite considerable difficulties in recovering weak lensing signals, the potential cosmological impact of the cosmic shear analysis has motivated several teams to devote efforts on imaging surveys designed for the measurement of the galaxy distortion produced by gravitational lensing, either by observing many independent small fields, like the VLT/FORS-I (Maoli et al. in preparation), the HST/STIS (Seitz et al. 1998), the WHT (Bacon et al. 2000), or by observing few intermediate to large fields, like the CFHT/CFH12K-UH8K (this work and Kaiser et al. 2000), the SDSS (Annis et al. 1998) and other ongoing surveys. In this paper we present the results of the analysis based on 2 square degrees obtained during previous independent observing runs at the CFHT with mixed I and V colors. This study is part of a our weak lensing survey carried out at CFHT (hereafter the DESCART project 1) which will cover 16 square degrees in four colors with the CFH12K camera. Though the survey is far from completion, data obtained during previous runs have been used jointly with the first observations of the DESCART survey that we did in May 1999 and in November 1999 in order to demonstrate that the technical issues can be overcome and to better prepare the next observations. This set of data permits us already to report on the detection of a cosmic shear signal.

In the following, we discuss the technique used to extract the cosmological signal and to measure its amplitude and show that systematic effects are well under control. The paper is organized as follows. Sect. 2 describes our data sets. Sect. 3 discusses the details of our PSF correction procedure and Sect. 4 presents the final results. Sect. 5 is devoted to the discussion of the residual systematics and their correction. Sect. 6 presents a preliminary quantitative comparison of our signal with numerical expectations of cosmological scenarios as derived from ray-tracing simulations. Conclusions are given in Sect. 7.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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