Systematic tests for position-dependent additive shear bias

TL;DR

The paper introduces new diagnostic tests to detect and quantify position-dependent additive shear biases in weak lensing data, crucial for accurate cosmic shear measurements.

Contribution

It presents novel methods for identifying and measuring position-dependent additive shear biases, including a grid-based ellipticity averaging technique and a spatial correlation analysis.

Findings

Detected residual additive biases in CFHTLenS and KiDS data.

Residual biases are smaller than shear correlation errors, unlikely to bias results.

Biases correlate with undersampled PSF variations in CFHTLenS fields.

Abstract

We present new tests to identify stationary position-dependent additive shear biases in weak gravitational lensing data sets. These tests are important diagnostics for currently ongoing and planned cosmic shear surveys, as such biases induce coherent shear patterns that can mimic and potentially bias the cosmic shear signal. The central idea of these tests is to determine the average ellipticity of all galaxies with shape measurements in a grid in the pixel plane. The distribution of the absolute values of these averaged ellipticities can be compared to randomized catalogues; a difference points to systematics in the data. In addition, we introduce a method to quantify the spatial correlation of the additive bias, which suppresses the contribution from cosmic shear and therefore eases the identification of a position-dependent additive shear bias in the data. We apply these tests to the publicly available shear catalogues from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) and the Kilo Degree Survey (KiDS) and find evidence for a small but non-negligible residual additive bias at small scales. As this residual bias is smaller than the error on the shear correlation signal at those scales, it is highly unlikely that it causes a significant bias in the published cosmic shear results of CFHTLenS. In CFHTLenS, the amplitude of this systematic signal is consistent with zero in fields where the number of stars used to model the PSF is higher than average, suggesting that the position-dependent additive shear bias originates from undersampled PSF variations across the image.