Calibration biases in measurements of weak lensing

TL;DR

This paper examines how non-linear calibration biases in weak lensing shear measurements can significantly bias cosmological parameters, emphasizing the need for bias-free methods or proper calibration verification.

Contribution

It demonstrates the impact of non-linear calibration relations on cosmological parameter estimates and highlights the importance of bias verification in weak lensing analyses.

Findings

Non-linear calibration introduces biases in shear measurements.

Bias magnitude depends on PSF width and measurement method.

Proper calibration verification is essential for reliable cosmological results.

Abstract

As recently shown by Viola et al., the common (KSB) method for measuring weak gravitational shear creates a non-linear relation between the measured and the true shear of objects. We investigate here what effect such a non-linear calibration relation may have on cosmological parameter estimates from weak lensing if a simpler, linear calibration relation is assumed. We show that the non-linear relation introduces a bias in the shear-correlation amplitude and thus a bias in the cosmological parameters Omega_matter and sigma_8. Its direction and magnitude depends on whether the point-spread function is narrow or wide compared to the galaxy images from which the shear is measured. Substantial over- or underestimates of the cosmological parameters are equally possible, depending also on the variant of the KSB method. Our results show that for trustable cosmological-parameter estimates from measurements of weak lensing, one must verify that the method employed is free from ellipticity-dependent biases or monitor that the calibration relation inferred from simulations is applicable to the survey at hand.