Seeing in the dark-II. cosmic shear in the sloan digital sky survey

Statistical weak lensing by large-scale structure-cosmic shear-is a promising cosmological tool, which has motivated the design of several large upcoming surveys. Here, we present a measurement of cosmic shear using co-added Sloan Digital Sky Survey (SDSS) imaging in 168 square degrees of the equatorial region, with r < 23.5 and i < 22.5, a source number density of 2.2 per arcmin² and mean redshift of z_med = 0.52. These co-adds were generated using a new method described in the companion Paper I that was intended to minimize systematic errors in the lensing measurement due to coherent point spread function anisotropies that are otherwise prevalent in the SDSS imaging data.We present measurements of cosmic shear out to angular separations of 2°, along with systematics tests that (combined with those from Paper I on the catalogue generation) demonstrate that our results are dominated by statistical rather than systematic errors. Assuming a cosmological model corresponding to Wilkinson Microwave Anisotropy Probe 7(WMAP7) and allowing only the amplitude of matter fluctuations σ₈ to vary, we find a best-fitting value of σ₈ = 0.636^+0.109 _-0.154 (1σ); without systematic errors this would be σ₈ = 0.636^+0.099 _-0.137 (1σ). Assuming a flatδcold dark matter model, the combined constraints with WMAP7 are σ₈ = 0.784^+0.028 _-0.026(1σ)^+0.055 _-0.054(2σ) and ω_mh² = 0.1303^+0.0047 _-0.0048(1σ)^+0.009 _-0.009(2σ); the 2σ error ranges are, respectively, 14 and 17 per cent smaller than WMAP7 alone.