Intrinsic alignment-lensing interference as a contaminant of cosmic shear

Abstract

Cosmic shear surveys have great promise as tools for precision cosmology, but can be subject to systematic errors including intrinsic ellipticity correlations of the source galaxies. The intrinsic alignments are believed to be small for deep surveys, but this is based on intrinsic and lensing distortions being uncorrelated. Here we show that the gravitational lensing shear and intrinsic shear need not be independent: correlations between the tidal field and the intrinsic shear cause the intrinsic shear of nearby galaxies to be correlated with the gravitational shear acting on more distant galaxies. We estimate the magnitude of this effect for two simple intrinsic-alignment models: one in which the galaxy ellipticity is linearly related to the tidal field, and one in which it is quadratic in the tidal field as suggested by tidal torque theory. The first model predicts a gravitational-intrinsic (GI) correlation that can be much greater than the intrinsic-intrinsic (II) correlation for broad redshift distributions, and that remains when galaxies pairs at similar redshifts are rejected. The second model, in its simplest form, predicts no gravitational-intrinsic correlation. In the first model, and assuming a normalization consistent with recently claimed detections of intrinsic correlations, we find that the GI correlation term can exceed the usual II term by >1 order of magnitude and the intrinsic correlation induced B-mode by 2 orders of magnitude. These interference effects can suppress the lensing power spectrum for a single broad redshift bin by of order 10% at z s 1 and 30% at z s 0:5. We conclude that, depending on the intrinsic-alignment model, the GI correlation may be the dominant contaminant of the lensing signal and can even affect cross spectra between widely separated bins. We describe two ways to constrain this effect, one based on density-shear correlations and one based on scaling of the cross correlation tomography signal with redshift.

CFHTLenS tomographic weak lensing cosmological parameter constraints: Mitigating the impact of intrinsic galaxy alignments

“…The broad agreement in results between the first estimates from numerical simulations (Heavens et al 2000;Croft & Metzler 2000), analytical studies (Catelan et al 2001;Crittenden et al 2001;Lee & Pen 2001), and the first low redshift observational constraints (Pen et al 2000;Brown et al 2002) resulted in a consistent picture; for deep weak lensing surveys, the contamination to the weak lensing signal was expected to be less than a few per cent effect. Hirata & Seljak (2004) were the first to highlight, however, the importance of also including the shear-shape correlations in the analysis, sγ , for galaxies that are separated by large physical distances along the line of sight. In this case the background galaxy experiences a shear γ caused by the foreground tidal gravitational field.…”

“…The recent two-bin tomographic analysis of the 154 square degree CFHTLenS, presented in Benjamin et al (2013), therefore combines the strategies of Huff et al (2011) and Schrabback et al (2010) to mitigate intrinsic alignment contamination. Using the linear tidal field intrinsic alignment model of Hirata & Seljak (2004), and following Bridle & King (2007) by fixing its amplitude to the observational constraints obtained by Brown et al (2002), they estimate the II and GI contamination to the cosmic shear measurement. They then limit their analysis to two broad high redshift bins with photometric redshifts 0.5 < z ph < 0.85 and 0.85 < z ph < 1.3 such that any contamination from intrinsic alignments is expected to be no more than a few per cent.…”

COSMOS: Three‐dimensional Weak Lensing and the Growth of Structure

“…Had it been significant on all scales, a likely explanation would have been cross-contamination of the bins by galaxies from other redshifts (the well-known degeneracy between low and high redshift from photo-z estimation is discussed in x 2.4). Had the covariance been equally evident in all three bins, likely explanations could have been interference of intrinsic alignments like those suggested by ( Hirata & Seljak 2004) and imperfect correction for PSF variation or DRIZZLE-related pixelization effects unaccounted for on small scales. In practice, the most likely explanation is a combination of several such effects, each at a low level.…”

Cross-correlating spectroscopic and photometric galaxy surveys

“…Galaxy bias which relates dark matter and galaxy counts is threated as a free parameter. The fiducial survey ignore bias stochasticity , shear intrinsic alignmants [59,20], other shear systematics [12] and uncertainties in photo-z distributions [91,87]. The DETF figure of merit is proportional to the inverse of the 1-σ contour area for the parameters (w 0 , w a ).…”

“…While galaxy shapes are intrinsically correlated due to the environment [59,20] and measuring shapes are difficult [113,129,89,69], weak lensing is potentially the most powerful probe in the next decade [2]. The broad lensing kernel reduce the importance of radial information and broad band photo-z is sufficient for measuring weak lensing.…”