Development of an unbiased cosmic shear estimator measured on galaxy images

TL;DR

This paper presents a new unbiased cosmic shear estimation method that measures galaxy shape moments directly from images, avoiding shape assumptions and calibration biases, with analytical and simulation-based bias correction.

Contribution

It introduces a shear estimator based on second moments that does not rely on galaxy shape assumptions, reducing calibration biases in cosmic shear measurements.

Findings

The method effectively reduces shear estimation bias.

Analytical noise bias correction matches simulation results.

Provides a fast, precise approach for unbiased cosmic shear measurement.

Abstract

Since cosmic shear was first observed in 2000, it has become a key cosmological probe and promises to deliver exquisite dark energy constraints. However, shear is inferred from coherent distortions of galaxy shapes, and the relation between galaxy ellipticities and gravitational shear is a serious potential source of bias. To address this, we are developing a shear estimation method that makes no assumption on galaxy shapes, in order to avoid the shortcomings of a simulation-based shear calibration. Our method relies on the estimation of second moments on the image, and the evaluation of how second moments respond to a shear applied to the coordinate system, without altering the image itself, at variance with the Metacalibration method. We also evaluate analytically the noise bias due to the non-linearity of the estimator, and confront it with the bias derived from noisy image simulations, which allows a fast and precise noise bias correction.