The shear power spectrum from the COMBO-17 survey

We perform a statistical weak lensing or “cosmic shear” analysis of the COMBO-17 survey – a unique dataset with shear quality R-band imaging and accurate photometric redshift estimates (σz = 0.05) for ∼ 90% of galaxies to mR ≤ 24.0. We undertake a full maximum likelihood analysis to measure directly from the data the weak lensing power spectra, C l , C ββ l and C l in 5 band powers from l = 400 to l = 10 , 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 is 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 normalisation of the matter power spectrum to be σ8 = (0.72 ± 0.09)(Ωm/0.3), where the errors quoted are 1-σ due to the intrinsic dispersion in galaxy ellipticities, cosmic and sampling variance. We have significantly reduced the usual additional uncertainty in the median redshift (zm) of the source galaxies by estimating zm directly 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 uncertainty in the resulting parameter constraints are reduced by more than a factor of 2 compared to 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 CMB. With these additional constraints, we measure the normalisation of the matter power spectrum to be σ8 = 0.73 +0.06 −0.03 and the matter density of the Universe to be Ωm = 0.27 +0.02 −0.01.