Detection of correlated galaxy ellipticities from CFHT data: first evidence for gravitational lensing by large-scale structures

We report the detection of a significant (5.5 sigma) excess of correlations between galaxy ellipticities at scales ranging from 0.5 to 3.5 arc-minutes. This detection of a gravitational lensing signal by large-scale structure was made using a composite high quality imaging sun ey of 6300 arcmin(2) obtained at the Canada France Hawaii Telescope (CFHT) with the UH8K and CFH12K panoramic CCD cameras. The amplitude of the excess correlation is 2.2 +/- 0.2% at 1 arcmin scale, in agreement with theoretical predictions of the lensing effect induced by large-scale structure. We provide a quantitative analysis of systematics which could contribute to the signal and show that the net effect is small and can be corrected for. In particular, we show that the spurious excess of correlations caused by the residual of the anisotropic Point Spread Function (PSF) correction is well below the measured signal. We show that the measured ellipticity correlations behave as expected for a gravitational shear signal. The relatively small size of our survey precludes tight constraints on cosmological models. However the data are in favor of cluster normalized cosmological models, and marginally reject Cold Dark Matter models with (Omega = 0.3, sigma(8) < 0.6) or (Omega = 1, sigma(8) = 1). The detection of cosmic sheer demonstrates the technical feasibility of using weak lensing surveys to measure dark matter clustering and the potential for cosmological parameter measurements, in particular with upcoming wide field CCD cameras.