Combining Size and Shape in Weak Lensing

TL;DR

This paper demonstrates that incorporating galaxy size measurements alongside shape in weak lensing analyses can significantly enhance cosmological parameter constraints, especially when systematic errors are minimized.

Contribution

It introduces a method to use galaxy size information, uncorrelated with shape, to improve dark energy constraints in weak lensing surveys.

Findings

Size measurement can be uncorrelated with ellipticity using sqrt(Area) as a size indicator.

Ignoring systematics, combining size and shape can improve the Dark Energy Figure of Merit by about 68%.

Potential improvements up to a factor of 4 in the Figure of Merit when systematics are properly managed.

Abstract

Weak lensing alters the size of images with a similar magnitude to the distortion due to shear. Galaxy size probes the convergence field, and shape the shear field, both of which contain cosmological information. We show the gains expected in the Dark Energy Figure of Merit if galaxy size information is used in combination with galaxy shape. In any normal analysis of cosmic shear, galaxy sizes are also studied, so this is extra statistical information comes for free and is currently unused. There are two main results in this letter: firstly, we show that size measurement can be made uncorrelated with ellipticity measurement, thus allowing the full statistical gain from the combination, provided that $\sqrt{Area}$ is used as a size indicator; secondly, as a proof of concept, we show that when the relevant modes are noise-dominated, as is the norm for lensing surveys, the gains are substantial, with improvements of about 68% in the Figure of Merit expected when systematic errors are ignored. An approximate treatment of such systematics such as intrinsic alignments and size-magnitude correlations respectively suggests that a much better improvement in the Dark Energy Figure of Merit of even a factor of ~4 may be achieved.