The removal of shear-ellipticity correlations from the cosmic shear signal via nulling techniques

Abstract

To render cosmic shear an astronomical tool of high precision, it is essential to eliminate systematic effects upon its signal, one of the most significant ones being correlations between the gravitational shear and the intrinsic ellipticity of source galaxies. Because of the crudeness of current models of intrinsic alignment, we have developed a model-independent, purely geometrical method to remove shear-ellipticity correlations. We eliminate the contributions to a tomographic cosmic shear signal that may be subject to contamination by shear-ellipticity correlations, making use of the characteristic dependence of these correlations on redshift. By introducing an appropriately chosen weight function to the lensing efficiency that nulls signals stemming from certain distances, new second-order measures of cosmic shear, free of intrinsic alignment, can be constructed. The loss of information induced by this nulling technique and the subsequent degradation of constraints on cosmological parameters is quantified in a likelihood analysis. Concerning the construction of optimal weight functions, good agreement is achieved between all approaches considered. For a survey divided into 20 redshift bins we find that the area of credible regions increases by 20% up to about 50% after the application of nulling, depending on the cosmological parameters considered. We demonstrate that due to an optimization of the weight functions nearly all information is contained in a small subset of the new second-order measures. The use of a significantly smaller number of redshift bins than 20 for the nulling considerably degrades parameter constraints under conservative assumptions, emphasizing the need for detailed redshift information.