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

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