Image analysis for cosmology: results from the GREAT10 Galaxy Challenge

Abstract

In this paper, we present results from the weak‐lensing shape measurement GRavitational lEnsing Accuracy Testing 2010 (GREAT10) Galaxy Challenge. This marks an order of magnitude step change in the level of scrutiny employed in weak‐lensing shape measurement analysis. We provide descriptions of each method tested and include 10 evaluation metrics over 24 simulation branches. GREAT10 was the first shape measurement challenge to include variable fields; both the shear field and the point spread function (PSF) vary across the images in a realistic manner. The variable fields enable a variety of metrics that are inaccessible to constant shear simulations, including a direct measure of the impact of shape measurement inaccuracies, and the impact of PSF size and ellipticity, on the shear power spectrum. To assess the impact of shape measurement bias for cosmic shear, we present a general pseudo‐Cℓ formalism that propagates spatially varying systematics in cosmic shear through to power spectrum estimates. We also show how one‐point estimators of bias can be extracted from variable shear simulations. The GREAT10 Galaxy Challenge received 95 submissions and saw a factor of 3 improvement in the accuracy achieved by other shape measurement methods. The best methods achieve sub‐per cent average biases. We find a strong dependence on accuracy as a function of signal‐to‐noise ratio, and indications of a weak dependence on galaxy type and size. Some requirements for the most ambitious cosmic shear experiments are met above a signal‐to‐noise ratio of 20. These results have the caveat that the simulated PSF was a ground‐based PSF. Our results are a snapshot of the accuracy of current shape measurement methods and are a benchmark upon which improvement can be brought. This provides a foundation for a better understanding of the strengths and limitations of shape measurement methods.

On scale-dependent cosmic shear systematic effects

“…This uses the steps shown in Kitching et al (2012) that take the Fourier transform of the field, rotate in Fourier space to an E/B-mode frame, and take the average of the Fourier transform of the real part in shells in angular wavenumber .…”

“…Hikage et al 2011) and galaxy shape estimation (see e.g. Kitching et al 2012), and the number density of stars (see e.g. Soumagnac et al 2013) can affect cosmic shear power spectrum estimation.…”

“…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.…”

“…These have typically looked at requirements either on an individual galaxy basis or under the assumption that systematic effects are not dependent on angular scale (or a random dependency). Some exceptions include the galaxy ellipticity measurement simulations in Kitching et al (2012) that investigated the change in power spectra from imperfect measurements, and Hoekstra (2004) who compared PSF modelling errors to the cosmic shear signal in data. Those studies that have included some scale dependence have made an assumption that all systematic effects have scale-dependent functional behaviour that mimics a cosmological signal, the requirement used being an integral over all scales used in the analysis.…”

Weak lensing measurements in simulations of radio images

“…The use of the shapelets method for radio data is well motivated, particularly by the Fourier invariance of the shapelet basis functions and the possibility of shape measurement directly in the uv plane. However, in weak lensing, there are a considerable number of methods for shape measurement, see for example Kitching et al (2012). Exploration of radio weak lensing should include a study of how these existing methods fare with radio image simulations.…”