The removal of shear-ellipticity correlations from the cosmic shear signal

Abstract

Aims. Cosmic shear, the gravitational lensing on cosmological scales, is regarded as one of the most powerful probes for revealing the properties of dark matter and dark energy. To fully utilize its potential, one has to be able to control systematic effects down to below the level of the statistical parameter errors. Particularly worrisome in this respect is the intrinsic alignment of galaxies, causing considerable parameter biases via correlations between the intrinsic ellipticities of galaxies and the gravitational shear, which mimic lensing. Since our understanding of the underlying processes of intrinsic alignment is still poor, purely geometrical methods are required to control this systematic. In an earlier work we proposed a nulling technique that downweights this systematic, only making use of its well-known redshift dependence. We assess the practicability of nulling, given realistic conditions on photometric redshift information. Methods. For several simplified intrinsic alignment models and a wide range of photometric redshift characteristics, we calculate an average bias before and after nulling. Modifications of the technique are introduced to optimize the bias removal and minimize the information loss by nulling. We demonstrate that one of the presented versions of nulling is close to optimal in terms of bias removal, given the high quality of photometric redshifts. Although the nulling weights depend on cosmology, being composed of comoving distances, we show that the technique is robust against an incorrect choice of cosmological parameters when calculating the weights. Moreover, general aspects such as the behavior of the Fisher matrix under parameter-dependent transformations and the range of validity of the bias formalism are discussed in an appendix. Results. Given excellent photometric redshift information, i.e. at least 10 bins with a dispersion σ ph 0.03, a negligible fraction of catastrophic outliers, and precise knowledge about the bin-wise redshift distributions as characterized by a scatter of 0.001 or less on the median redshifts, one version of nulling is capable of reducing the shear-intrinsic ellipticity contamination by at least a factor of 100. Alternatively, we describe a robust nulling variant which suppresses the systematic signal by about 10 for a very broad range of photometric redshift configurations, provided basic information about σ ph in each of 10 photometric redshift bins is available. Irrespective of the photometric redshift quality, a loss of statistical power is inherent to nulling, which amounts to a decrease of the order 50% in terms of our figure of merit under conservative assumptions.

Constraints on intrinsic alignment contamination of weak lensing surveys using the MegaZ-LRG sample

“…With the Fisher matrix, one can calculate the bias on a cosmological parameter via (e.g. Kim et al 2004;Huterer & Takada 2005;Huterer et al 2006;Taylor et al 2007;Amara & Réfrégier 2008;Kitching et al 2009;Joachimi & Schneider 2009) …”

Weak lensing power spectra for precision cosmology

“…On the observational side, this requires accurate information on the redshift distribution of source galaxies (Ma et al 2006) and precise measurements of galaxy shapes which correct for observational systematics such as pixelization, noise, blurring by seeing and a spatially variable point spread function (see Massey et al 2007a;Bridle et al 2009). On the theoretical side, astrophysical contaminants, like source lens clustering (Bernardeau et al 1997;Schneider et al 2002), intrinsic alignment (King & Schneider 2003) and the correlation between the gravitational shear and intrinsic ellipticities of galaxies (Hirata & Seljak 2004;King 2005;Joachimi & Schneider 2008;Zhang 2010;Joachimi & Schneider 2009), need to be understood and removed. The prediction of lensing observables also requires precise models of the non-linear matter power spectrum and models for the relation between lensing distortion and large scale matter distribution which go beyond linear theory.…”

Spatial curvature endgame: Reaching the limit of curvature determination

“…g ðχÞ is the distribution of source galaxies in each redshift bin i, and accounts for photometric redshifts using the method described in [36]. The photo-z bias is assumed to be zero, while the photo-z scatter, σ z , is used to determine the number of redshift bins such that Δz ¼ 3σ z ð1 þ zÞ.…”