KiDS-Legacy: Cosmological constraints from cosmic shear with the complete Kilo-Degree Survey (2503.19441v1)

Abstract: We present cosmic shear constraints from the completed Kilo-Degree Survey (KiDS), where the cosmological parameter $S_8\equiv\sigma_8\sqrt{\Omega_{\rm m}/0.3} = 0.815^{+0.016}_{-0.021}$, is found to be in agreement ($0.73\sigma$) with results from the Planck Legacy cosmic microwave background experiment. The final KiDS footprint spans $1347$ square degrees of deep nine-band imaging across the optical and near-infrared, along with an extra $23$ square degrees of KiDS-like calibration observations of deep spectroscopic surveys. Improvements in our redshift distribution estimation methodology, combined with our enhanced calibration data and multi-band image simulations, allow us to extend our lensed sample out to a photometric redshift of $z_{\rm B}\leq2.0$. Compared to previous KiDS analyses, the increased survey area and redshift depth results in a $\sim32\%$ improvement in constraining power in terms of $\Sigma_8\equiv\sigma_8\left(\Omega_{\rm m}/0.3\right)^\alpha = 0.821^{+0.014}_{-0.016}$, where $\alpha = 0.58$ has been optimised to match the revised degeneracy direction of $\sigma_8$ and $\Omega_{\rm m}$. We adopt a new physically motivated intrinsic alignment model that depends jointly on the galaxy sample's halo mass and spectral type distributions, and that is informed by previous direct alignment measurements. We also marginalise over our uncertainty on the impact of baryon feedback on the non-linear matter power spectrum. Comparing to previous KiDS analyses, we conclude that the increase seen in $S_8$ primarily results from our improved redshift distribution estimation and calibration, as well as new survey area and improved image reduction. Our companion paper St\"olzner et al. (submitted) presents a full suite of internal and external consistency tests, finding the KiDS-Legacy data set to be the most internally robust sample produced by KiDS to date.

Cosmological Analysis of Cosmic Shear Data from the Kilo-Degree Survey

The paper presents an in-depth analysis of cosmic shear data collected from the Kilo-Degree Survey (KiDS) to explore constraints on cosmological parameters, specifically focusing on the matter distribution and dark energy in the universe. This paper utilizes extensive imaging data from KiDS, which incorporates nine broadband filters spanning optical and near-infrared wavelengths, providing a robust data set across a 1347 square degree area, with an additional 23 square degrees used for calibration from deep spectroscopic surveys.

Methodology

The analysis employs cosmic shear as a probe, utilizing novel enhancements in redshift distribution estimation, combined with multi-band image simulations, to extend the photometric redshift range to $z_{\rm B} \leq 2.0$. The improvements in survey depth and area resulted in a constraining power enhancement of approximately 32%, quantified by the parameter $\Sigma_8 = \sigma_8 (\Omega_{\rm m}/0.3)^\alpha$ with $\alpha = 0.58$, which indicates a revised degeneracy direction of $\sigma_8$ and $\Omega_{\rm m}$ due to the higher redshift data.

The paper adopts an intrinsic alignment model that jointly considers galaxy sample halo masses and spectral types, validating these against previous direct measurements. The analysis marginalizes over uncertainties stemming from baryonic feedback on the non-linear matter power spectrum to address any potential biases in the derived cosmological parameters.

Key Results

A major finding is that the $S_8 \equiv \sigma_8 \sqrt{\Omega_{\rm m}/0.3}$ parameter is measured to be $0.815^{+0.016}_{-0.021}$, which is consistent within $0.73\sigma$ with results from the Planck Legacy project. The utilization of an advanced intrinsic alignment model illustrates the minimal impact of these effects on cosmological conclusions, enabling more confident estimates of fundamental parameters.

Furthermore, the analysis highlights the statistical precision of the stage-III surveys like KiDS, indicating a clear role for cosmic shear as a competitive tool in cosmology alongside traditional methods such as the Cosmic Microwave Background (CMB) and Baryonic Acoustic Oscillations (BAO).

Implications and Future Directions

This research underscores the KiDS project's methodology, asserting that improvements in redshift calibration and intrinsic alignment modeling significantly enhance the reliability of weak-lensing cosmological inferences. The implications are profound for future cosmic shear analyses, particularly with upcoming stage-IV surveys such as ESA's Euclid and NASA's Roman Space Telescope. The paper also sets a benchmark for data consistency and methodological rigor that is vital for unraveling the persistent $S_8$ tension observed between high- and low-redshift probes.

The paper forecasts that ongoing and future surveys, with even finer precision and broader coverage, will yield insights able to challenge the standard cosmological model further — probing the potential necessity for new physics beyond $\Lambda$CDM. It is crucial to continue refining statistical and systematic error controls, particularly as they pertain to baryonic and intrinsic alignments, to leverage the full data utility of forthcoming observational campaigns.

In conclusion, the KiDS analysis illustrates the mature capabilities of cosmic shear as a cosmological tool. It demonstrates that with careful modeling of systematic effects, cosmic shear can yield competitive constraints on the parameters governing the universe's large-scale structure and evolution, aligning closely with independent measurements from other cosmological observations.