KiDS+VIKING-450: Cosmic shear tomography with optical+infrared data

TL;DR

KV450 presents a wide-field cosmic shear tomography analysis that combines KiDS optical data with VIKING infrared imaging to achieve significantly improved photometric redshift calibrations across five tomographic bins. Using a robust DIR redshift calibration with extensive spectroscopic coverage and redundant cross-checks (including CC and OQE) and a state-of-the-art theoretical pipeline (CLASS+HMCode+MontePython), the study finds S_8 = 0.737_{-0.036}^{+0.040} under a flat $Λ$CDM model, with no significant B-modes and a good fit to the data. The results remain in tension with Planck CMB measurements at ~2.3σ, and the tension can be shifted modestly if the redshift calibration relies on the COSMOS-2015 photo-$z$ catalogue, highlighting redshift distribution uncertainties as a key systematic for future surveys. KV450 thus represents a milestone in combining optical+NIR data for cosmic shear, demonstrates robust redshift calibration, and underscores the central role of redshift distributions in precision weak lensing cosmology. The work also provides comprehensive robustness tests, an updated covariance treatment, and data products to enable further cross-survey comparisons and methodological refinements.

Abstract

We present a tomographic cosmic shear analysis of the Kilo-Degree Survey (KiDS) combined with the VISTA Kilo-Degree Infrared Galaxy Survey (VIKING). This is the first time that a full optical to near-infrared data set has been used for a wide-field cosmological weak lensing experiment. This unprecedented data, spanning $450~$deg$^2$, allows us to improve significantly the estimation of photometric redshifts, such that we are able to include robustly higher-redshift sources for the lensing measurement, and - most importantly - solidify our knowledge of the redshift distributions of the sources. Based on a flat $Λ$CDM model we find $S_8\equivσ_8\sqrt{Ω_{\rm m}/0.3}=0.737_{-0.036}^{+0.040}$ in a blind analysis from cosmic shear alone. The tension between KiDS cosmic shear and the Planck-Legacy CMB measurements remains in this systematically more robust analysis, with $S_8$ differing by $2.3σ$. This result is insensitive to changes in the priors on nuisance parameters for intrinsic alignment, baryon feedback, and neutrino mass. KiDS shear measurements are calibrated with a new, more realistic set of image simulations and no significant B-modes are detected in the survey, indicating that systematic errors are under control. When calibrating our redshift distributions by assuming the 30-band COSMOS-2015 photometric redshifts are correct (following the Dark Energy Survey and the Hyper Suprime-Cam Survey), we find the tension with Planck is alleviated. The robust determination of source redshift distributions remains one of the most challenging aspects for future cosmic shear surveys.

Figures (14)

• Figure 1: Redshift distribution estimates for the five tomographic bins used in the KV450 cosmic shear analysis with the DIR technique. The uncertainties shown correspond to the 68% confidence intervals as estimated from a spatial bootstrap resampling of the spec-$z$ calibration sample.
• Figure 2: Map of the predicted $c_1$ term at $r=24$ based on the findings of Hoekstra et al. (in prep.) about electronic effects in the OmegaCam instrument (the $c_2$ pattern is insignificant). We calculate the corresponding 2-point shear correlation function of this pattern and add this to our model via a free nuisance amplitude $A_c$.
• Figure 3: KV450 2-point shear correlation functions $\xi_+$ (upper-left) and $\xi_-$ (lower-right) plotted as $\theta\times\xi_\pm$. The errors shown represent the square root of the diagonal of the analytical covariance matrix. These errors are significantly correlated between scales and redshift bins. The solid red line corresponds to the best-fit (maximum likelihood) fiducial model from Sect. \ref{['sec:results']} including baryon feedback, intrinsic alignments, and all corrections for observational biases.
• Figure 4: Marginalised posterior contours (inner 68% confidence level, outer 95% confidence level) in the $\Omega_{\rm m}$-$\sigma_8$ plane (left) and the $\Omega_{\rm m}$-$S_8$ plane (right) for the fiducial KV450 setup (blue), the optical-only KiDS-450 analysis from hildebrandt/etal:2017 (green), DESy1 using cosmic shear only troxel/etal:2018a, HSC-DR1 cosmic shear hikage/etal:2018, and the Planck-Legacy analysis planck/cosmo:2018.
• Figure 5: Same as Fig. \ref{['fig:Om_s8_fidDz']} but for the most extreme alternative redshift distributions described in Sect. \ref{['sec:DIR']}. Brown contours correspond to the redshift distributions with the highest $S_8$ (DIR-C15) that we argue in Sect. \ref{['sec:DIR']} might produce a biased result. The pink contours correspond to the redshift distributions with the lowest $S_8$ (OQE-shift).