Spin‐induced Galaxy Alignments and Their Implications for Weak‐Lensing Measurements

Crittenden, Robert; Natarajan, Priyamvada; Pen, Ue-Li; Theuns, Tom

doi:10.1086/322370

Cited by 296 publications

(440 citation statements)

References 37 publications

(52 reference statements)

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“…The alignments that we measure in the nonlinear regime are evolved versions of shape correlations built into the initial linear density field (Catelan et al 2001;Crittenden et al 2001). The form of ellipticity correlations calculated by Crittenden et al is similar to the shape correlations of Figure 7, with a constant core region and a power-law fall-off at large separations.…”

Section: Spatial Correlations Of Shapessupporting

confidence: 71%

Shapes and Alignments of Galaxy Cluster Halos

Kasun

Evrard

2005

ApJ

186

244

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We present distribution functions and mark correlations of the shapes of massive dark matter halos derived from Hubble volume simulations of a ÃCDM universe. We measure both position and velocity shapes within spheres that encompass a mean density 200 times the critical value and calibrate small-N systematic errors using Poisson realizations of isothermal spheres and higher resolution simulations. For halos more massive than 3 ; 10 14 h À1 M , the shape distribution function peaks at (minor/major, intermediate/major) axial ratios of (0.64, 0.76) in position and is rounder in velocity, peaking at (0.72, 0.82). Halo shapes are rounder at lower mass and/or redshift; the mean minoraxis ratio in position follows hc/ai(M ; z) ¼ c 15;0 ½1 À ln (M/10 15 h À1 M ) (1 þ z) À , with c 15;0 ¼ 0:631 AE 0:001, ¼ 0:023 AE 0:002, and ¼ 0:086 AE 0:004. Position and velocity principal axes are well aligned in direction, with median alignment angle 22 , and the axial ratios in these spaces are correlated in magnitude. We investigate mark correlations of halo pair orientations using two measures: a simple scalar product shows !1% alignment extending to 30 h À1 Mpc, while a filamentary statistic exhibits nonrandom alignment extending to scales $200 h À1 Mpc, 10 times the sample two-point correlation length and well into the regime of negative two-point correlation. Shapes of cluster halos are little affected by the large-scale environment; the distribution of supercluster member minor-axis ratios differs from that of the general population at only the few percent level.

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Section: Spatial Correlations Of Shapessupporting

confidence: 71%

Shapes and Alignments of Galaxy Cluster Halos

Kasun

Evrard

2005

ApJ

186

244

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“…Another possible systematic error is intrinsic (i.e. not lensing-induced) correlations among the ellipticities of neighboring source galaxies [25][26][27][28][29][30][31][32][33][34], which could arise if the galaxy ellipticities are affected by large-scale tidal fields. This systematic error is particularly worrisome because it lies outside the control of the observer, and is dependent upon the poorly understood physics of galaxy formation.…”

Section: Introductionmentioning

confidence: 99%

Intrinsic alignment-lensing interference as a contaminant of cosmic shear

2004

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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.

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“…Moreover, a matter overdensity can align a close-by galaxy and at the same time contribute to the lensing signal of a background object, which results in non-zero correlations between gravitational shear and intrinsic ellipticities or a GI term (Hirata & Seljak 2004, HS04 hereafter). The alignment of dark matter haloes, resulting from external tidal forces, has been subject to extensive study, both analytic and numerical (Croft & Metzler 2000;Heavens et al 2000;Lee & Pen 2000;Catelan et al 2001;Crittenden et al 2001;Jing 2002;Mackey et al 2002;HS04;Bridle & Abdalla 2007;Schneider & Bridle 2009). The galaxies in turn are assumed to align with the angular momentum vector (in the case of spiral galaxies) or the shape of their host halo (in the case of elliptical galaxies), which is suggested by the observed correlations of galaxy spins (e.g.…”

Section: Gg II Gimentioning

confidence: 99%

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

Joachimi¹,

Schneider²

2009

A&A

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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.

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Spin‐induced Galaxy Alignments and Their Implications for Weak‐Lensing Measurements

Cited by 296 publications

References 37 publications

Shapes and Alignments of Galaxy Cluster Halos

Shapes and Alignments of Galaxy Cluster Halos

Intrinsic alignment-lensing interference as a contaminant of cosmic shear

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

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