IM3SHAPE: A maximum-likelihood galaxy shear measurement code for cosmic gravitational lensing

TL;DR

im3shape is a publicly available maximum-likelihood galaxy shape measurement code designed for weak gravitational lensing, capable of analyzing noisy images and meeting current survey requirements, with some limitations for future surveys.

Contribution

The paper introduces im3shape, a new galaxy shape measurement code that incorporates a bulge-plus-disc model and demonstrates its effectiveness on simulated data for cosmic shear surveys.

Findings

Achieves sub-two percent multiplicative bias on GREAT08 simulations.

Meets current survey requirements with negligible additive bias.

Falls short of sub-percent bias needed for future surveys due to noise bias and model mismatch.

Abstract

We present and describe im3shape, a new publicly available galaxy shape measurement code for weak gravitational lensing shear. im3shape performs a maximum likelihood fit of a bulge-plus-disc galaxy model to noisy images, incorporating an applied point spread function. We detail challenges faced and choices made in its design and implementation, and then discuss various limitations that affect this and other maximum likelihood methods. We assess the bias arising from fitting an incorrect galaxy model using simple noise-free images and find that it should not be a concern for current cosmic shear surveys. We test im3shape on the GREAT08 Challenge image simulations, and meet the requirements for upcoming cosmic shear surveys in the case that the simulations are encompassed by the fitted model, using a simple correction for image noise bias. For the fiducial branch of GREAT08 we obtain a negligible additive shear bias and sub-two percent level multiplicative bias, which is suitable for analysis of current surveys. We fall short of the sub-percent level requirement for upcoming surveys, which we attribute to a combination of noise bias and the mis-match between our galaxy model and the model used in the GREAT08 simulations. We meet the requirements for current surveys across all branches of GREAT08, except those with small or high noise galaxies, which we would cut from our analysis. Using the GREAT08 metric we we obtain a score of Q=717 for the usable branches, relative to the goal of Q=1000 for future experiments. The code is freely available from https://bitbucket.org/joezuntz/im3shape