KiDS-1000: cosmic shear with enhanced redshift calibration

TL;DR

This paper improves cosmic shear analysis in KiDS-1000 by expanding and enhancing redshift calibration using self-organising maps and additional photometric redshift data, leading to more precise cosmological constraints.

Contribution

It introduces an expanded redshift calibration sample with self-organising maps and photometric redshifts, improving the accuracy of cosmic shear measurements in KiDS-1000.

Findings

Measured S8 = 0.748 with uncertainties, consistent with previous KiDS results.

Redshift calibration is robust across different sample subsets, within 0.5σ.

Including photometric redshifts increases calibration coverage by 6%.

Abstract

We present a cosmic shear analysis with an improved redshift calibration for the fourth data release of the Kilo-Degree Survey (KiDS-1000) using self-organising maps (SOMs). Compared to the previous analysis of the KiDS-1000 data, we expand the redshift calibration sample to more than twice its size, now consisting of data of 17 spectroscopic redshift campaigns, and significantly extending the fraction of KiDS galaxies we are able to calibrate with our SOM redshift methodology. We then enhance the calibration sample with precision photometric redshifts from COSMOS2015 and the Physics of the Accelerated Universe Survey (PAUS), allowing us to fill gaps in the spectroscopic coverage of the KiDS data. Finally we perform a Complete Orthogonal Sets of E/B-Integrals (COSEBIs) cosmic shear analysis of the newly calibrated KiDS sample. We find $S_8 = 0.748_{-0.025}^{+0.021}$, which is in good agreement with previous KiDS studies and increases the tension with measurements of the cosmic microwave background to 3.4{\sigma}. We repeat the redshift calibration with different subsets of the full calibration sample and obtain, in all cases, agreement within at most 0.5{\sigma} in $S_8$ compared to our fiducial analysis. Including additional photometric redshifts allows us to calibrate an additional 6 % of the source galaxy sample. Even though further systematic testing with simulated data is necessary to quantify the impact of redshift outliers, precision photometric redshifts can be beneficial at high redshifts and to mitigate selection effects commonly found in spectroscopically selected calibration samples.