2003MNRAS.341..100B

We perform a statistical weak lensing or `cosmic shear' analysis of the COMBO-17 survey - a unique data set with shear quality R -band imaging and accurate photometric redshift estimates (σ<sub>z</sub>= 0.05) for ∼90 per cent of galaxies to m<sub>R</sub>≤ 24.0. We undertake a full maximum likelihood analysis to measure directly from the data the weak lensing power spectra, C<sup>κκ</sup><sub>ℓ</sub>, C<sup>ββ</sup><sub>ℓ</sub>and C<sup>κβ</sup><sub>ℓ</sub> in five band powers from ℓ= 400 to ℓ= 10<sup>4</sup>, where κ is the usual lens convergence and β is an odd-parity `curl' component of the shear signal. We find a strong measurement of the convergence power over five fields. The non-gravitational β-field has a much lower significance, indicating our data are free of major systematics, while the cross-correlation of κ and β is consistent with zero. We have also calculated the shear correlation functions and variance over a range of scales between 0.5 and 20 arcmin. Our measurements of minimal star-galaxy correlations and cross-correlations between galaxy components provide further evidence that any systematics are negligible.

In addition, we have used our results to measure cosmological parameters, constraining the normalization of the matter power spectrum to be σ₈=(0.72±0.09)(Ω_m/0.3)^–0.49, where the errors quoted are 1σ due to the intrinsic dispersion in galaxy ellipticities, cosmic and sampling variance. We have significantly reduced the usual additional uncertainty in the median redshift (z_m) of the source galaxies by estimating z_mdirectly from our data using accurate photometric redshift information from the COMBO-17 multi-band wide-field survey. To demonstrate the power of accurate redshift information, we have also measured parameters from a shear analysis of only those galaxies for which accurate redshift estimates are available. In this case, we have eliminated the uncertainty in the redshift distribution of sources and we show that the uncertainties in the resulting parameter constraints are reduced by more than a factor of 2 compared with the typical uncertainties found in cosmic shear surveys to date. Finally, we combine our parameter measurements with constraints from the 2dF Galaxy Redshift Survey and with those from the cosmic microwave background. With these additional constraints, we measure the normalization of the matter power spectrum to be σ₈= 0.73^+0.06_–0.03and the matter density of the Universe to be Ω_m= 0.27^+0.02_–0.01.