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We report a detection of the coherent distortion of faint galaxies arisingfrom gravitational lensing by foreground structures. This ``cosmic shear'' ispotentially the most direct measure of the mass power spectrum, as it isunaffected by poorly-justified assumptions made concerning the biasing of thedistribution. Our detection is based on an initial imaging study of 14separated 8' x 16' fields observed in good, homogeneous conditions with theprime focus EEV CCD camera of the 4. 2m William Herschel Telescope. We detect anrms shear of 1. 6 0n 8' x 8' cells, with a significance of 3. 4 sigma. Wecarefully justify this detection by quantifying various systematic effects andcarrying out extensive simulations of the recovery of the shear signal fromartificial images defined according to measured instrument characteristics. Wealso verify our detection by computing the cross-correlation between the shearin adjacent cells. Including (gaussian) cosmic variance, we measure the shearvariance to be (0. 016) ² plus/minus (0. 012) ² plus/minus (0. 006) ², where these1 sigma errors correspond to statistical and systematic uncertainties, respectively. Our measurements are consistent with the predictions ofcluster-normalised CDM models (within 1 sigma) but a COBE-normalised SCDM modelis ruled out at the 3. 0 sigma level. For the currently-favoured Lambda-CDMmodel (with Omegaₘ = 0. 3), our measurement provides a normalisation of themass power spectrum of sigma₈ = 1. 5 plus/minus 0. 5, fully consistent with thatderived from cluster abundances. Our result demonstrates that ground-basedtelescopes can, with adequate care, be used to constrain the mass powerspectrum on various scales. The present results are limited mainly by cosmicvariance, which can be overcome in the near future with more observations.
Bacon et al. (Sun,) studied this question.