The intrinsic alignment of galaxies and its impact on weak gravitational lensing in an era of precision cosmology

Abstract

Please cite this article as: M.A. Troxel, M. Ishak, The intrinsic alignment of galaxies and its impact on weak gravitational lensing in an era of precision cosmology, Physics Reports (2014), http://dx. AbstractThe wealth of incoming and future cosmological observations will allow us to map out the structure and evolution of the observable universe to an unprecedented level of precision. Among these observations is the weak gravitational lensing of galaxies, e.g., cosmic shear that measures the minute distortions of background galaxy images by intervening cosmic structure. Weak lensing and cosmic shear promise to be a powerful probe of astrophysics and cosmology, constraining models of dark energy, measuring the evolution of structure in the universe, and testing theories of gravity on cosmic scales. However, the intrinsic alignment of galaxies -their shape and orientation before being lensed -may pose a great challenge to the use of weak gravitational lensing as an accurate cosmological probe, and has been identified as one of the primary physical systematic biases in cosmic shear studies. Correlations between this intrinsic alignment and the lensing signal can persist even for large physical separations, and isolating the effect of intrinsic alignment from weak lensing is not trivial. A great deal of work in the last two decades has been devoted to understanding and characterizing this intrinsic alignment, which is also a direct and complementary probe of structure formation and evolution in its own right. In this review, we report in a systematic way the state of our understanding of the intrinsic alignment of galaxies, with a particular emphasis on its large-scale impact on weak lensing measurements and methods for its isolation or mitigation. We begin with an introduction to the use of cosmic shear as a probe for cosmology and describe the various physical contributions by intrinsic alignment to the shear or convergence 2-and 3-point correlations. We then review developments in the modeling of the intrinsic alignment signal, including a trend toward attempting to incorporate more accurate nonlinear and single halo effects. The impact on cosmological constraints by the intrinsic alignment of galaxies is also outlined based on these models. We then summarize direct measurements of the large-scale intrinsic alignment signal in various surveys and discuss their constraints on models of intrinsic alignment, as well as progress in utilizing numerical simulations of structure formation to further our understanding of intrinsic alignment. Finally, we outline the development of a variety of mitigation techniques for reducing the impact of the intrinsic alignment contamination on weak lensing signals both within a galaxy data set and between complementary probes of gravitational lensing. The methodology and projected impact of these techniques is discussed for both 2-and 3-point correlations. We conclude by presenting a summary and outlook on the state of intrinsic alignment study and its impact in ongoing and planned ...

Joint measurement of lensing–galaxy correlations using SPT and DES SV data

“…Giannantonio et al (2016) have considered how several different systematics -in both DES and SPT data -may impact the measurement of w κg (θ), finding no evidence for significant contamination. Finally, we mention that if there is overlap between the source and tracer galaxies in redshift, then intrinsic alignment effects (Troxel & Ishak 2015) can lead to a distinct signature in the measured γT that could in principle be constrained using the joint w κg (θ) and w γ T g (θ) observable. Since w γ T g (θ) would be affected by intrinsic alignments while w κg (θ) is not, the joint lensing observable is a potentially attractive probe of these effects.…”

On scale-dependent cosmic shear systematic effects

“…The measurement from data is complicated in several respects: even without the shear effect galaxy images would appear elliptical, there are 'intrinsic' ellipticity correlations (see Troxel & Ishak 2014, Joachimi et al 2015 for reviews); the measurement of ellipticity is affected by noise (Viola, Kitching & Joachimi 2014) and modelling uncertainties (if a galaxy model is used, see for example Voigt & Bridle 2010;Kitching et al 2012); telescopes have an impulse response, or point spread function (PSF) that blurs images (and, for ground-based telescopes, atmospheric effects further blur the images), and the PSF is typically also elliptical to some degree; CCD detectors themselves can also degrade ellipticity as a result of radiation damage charge transfer inefficiency (CTI), manufacturing errors, or properties inherent to the devices. Therefore, the control of systematic effects in experiments that wish to measure cosmic shear need to be characterized and accounted for in a rigorous manner.…”

Modelling the impact of intrinsic size and luminosity correlations on magnification estimation

“…For a comparison on small scales, we apply the so-called nonlinear alignment (NLA) model (e.g. Troxel & Ishak (2015)), using the non-linear evolution of the matter power spectrum calculated with HALOFIT from Smith et al (2003) and revised by Takahashi et al (2012).…”

“…These intrinsic alignments produce a signal that can mimic the effect of weak lensing and bias cosmological parameter constraints. A review of the various intrinsic alignment models and the methods to assess the contamination can be found in Troxel & Ishak (2015). Heavens et al (2013) have shown that adding cosmic magnification via size distortions can help to increase the constraining power compared to cosmic shear measurements on their own; they also demonstrated that size measurements can be made largely uncorrelated with shape measurements if the square root of the area of the galaxy image is used as size estimator.…”