KiDS-Legacy calibration: unifying shear and redshift calibration with the SKiLLS multi-band image simulations

TL;DR

This paper introduces SKiLLS, a comprehensive multi-band simulation suite for KiDS that improves shear and redshift calibration accuracy by incorporating realistic galaxy properties, PSF variations, and blending effects.

Contribution

The paper presents SKiLLS, a novel simulation framework that unifies shear and redshift calibration for KiDS using realistic, high-fidelity multi-band image simulations.

Findings

Redshift-bias correlation linked to blending effects.

Small but noticeable impact of PSF modelling errors on shear bias.

Robustness of lensfit shape measurement algorithm within KiDS requirements.

Abstract

We present SKiLLS, a suite of multi-band image simulations for the weak lensing analysis of the complete Kilo-Degree Survey (KiDS), dubbed KiDS-Legacy analysis. The resulting catalogues enable joint shear and redshift calibration, enhancing the realism and hence accuracy over previous efforts. To create a large volume of simulated galaxies with faithful properties and to a sufficient depth, we integrated cosmological simulations with high-quality imaging observations. We also improved the realism of simulated images by allowing the point spread function (PSF) to differ between CCD images, including stellar density variations and varying noise levels between pointings. Using realistic variable shear fields, we accounted for the impact of blended systems at different redshifts. Although the overall correction is minor, we found a clear redshift-bias correlation in the blending-only variable shear simulations, indicating the non-trivial impact of this higher-order blending effect. We also explored the impact of the PSF modelling errors and found a small yet noticeable effect on the shear bias. Finally, we conducted a series of sensitivity tests, including changing the input galaxy properties. We conclude that our fiducial shape measurement algorithm, lensfit, is robust within the requirements of lensing analyses with KiDS. As for future weak lensing surveys with tighter requirements, we suggest further investments in understanding the impact of blends at different redshifts, improving the PSF modelling algorithm and developing the shape measurement method to be less sensitive to the galaxy properties.