A bias in cosmic shear from galaxy selection: results from ray-tracing simulations

TL;DR

This paper uncovers a new systematic bias in cosmic shear measurements caused by galaxy selection, particularly the rejection of close pairs, which affects cosmological parameter estimates in upcoming surveys.

Contribution

The study identifies a previously unknown bias from galaxy selection in cosmic shear, quantifies its impact using simulations, and proposes a weighting scheme to mitigate it.

Findings

Bias causes a several percent scale-dependent shift in shear correlation functions.

The bias leads to cosmological parameter estimation errors of similar magnitude.

A weighting scheme can significantly reduce the identified bias.

Abstract

We identify and study a previously unknown systematic effect on cosmic shear measurements, caused by the selection of galaxies used for shape measurement, in particular the rejection of close (blended) galaxy pairs. We use ray-tracing simulations based on the Millennium Simulation and a semi-analytical model of galaxy formation to create realistic galaxy catalogues. From these, we quantify the bias in the shear correlation functions by comparing measurements made from galaxy catalogues with and without removal of close pairs. A likelihood analysis is used to quantify the resulting shift in estimates of cosmological parameters. The filtering of objects with close neighbours (a) changes the redshift distribution of the galaxies used for correlation function measurements, and (b) correlates the number density of sources in the background with the density field in the foreground. This leads to a scale-dependent bias of the correlation function of several percent, translating into biases of cosmological parameters of similar amplitude. This makes this new systematic effect potentially harmful for upcoming and planned cosmic shear surveys. As a remedy, we propose and test a weighting scheme that can significantly reduce the bias.