[Paper Review] KiDS-1000: Cosmology with improved cosmic shear measurements

Shangrong Li, Henk Hoekstra+13 more|arXiv (Cornell University)|Jun 19, 2023

Galaxies: Formation, Evolution, Phenomena5 references4 citations

TL;DR

This paper presents improved cosmological constraints from the KiDS-1000 cosmic shear survey using enhanced galaxy shape measurements and advanced shear calibration via multi-band image simulations. It reports $S_8 = 0.776^{+0.029+0.002}_{-0.027-0.003}$, showing a $\sim2.3\sigma$ tension with Planck CMB results, while systematic uncertainties contribute ~8% to the final $S_8$ error budget.

ABSTRACT

We present refined cosmological parameter constraints derived from a cosmic shear analysis of the fourth data release of the Kilo-Degree Survey (KiDS-1000). Our main improvements include enhanced galaxy shape measurements made possible by an updated version of the lensfit code and improved shear calibration achieved with a newly developed suite of multi-band image simulations. Additionally, we incorporated recent advancements in cosmological inference from the joint Dark Energy Survey Year 3 and KiDS-1000 cosmic shear analysis. Assuming a spatially flat standard cosmological model, we constrain $S_8\equivσ_8(Ω_{ m m}/0.3)^{0.5} = 0.776_{-0.027-0.003}^{+0.029+0.002}$, where the second set of uncertainties accounts for the systematic uncertainties within the shear calibration. These systematic uncertainties stem from minor deviations from realism in the image simulations and the sensitivity of the shear measurement algorithm to the morphology of the galaxy sample. Despite these changes, our results align with previous KiDS studies and other weak lensing surveys, and we find a ${\sim}2.3σ$ level of tension with the Planck cosmic microwave background constraints on $S_8$.

Motivation & Objective

• To refine cosmological parameter constraints from cosmic shear measurements in the KiDS-1000 survey using improved galaxy shape measurement and shear calibration.
• To reduce systematic uncertainties in weak lensing analysis, particularly those arising from image simulation realism and shape measurement algorithm sensitivity.
• To assess the impact of intrinsic alignment (IA) modeling on $S_8$ constraints using multiple IA model variants.
• To evaluate the consistency of results with previous KiDS and DES cosmic shear analyses and with Planck CMB constraints.
• To quantify the contribution of systematic uncertainties to the final $S_8$ error budget, especially from shear calibration residuals.

Proposed method

• Utilized an updated version of the lens fit code to improve galaxy shape measurements in the KiDS-1000 data release.
• Developed a new suite of multi-band image simulations to enhance shear calibration and reduce systematic biases.
• Applied a forward modeling approach with local minimization to estimate the impact of residual shear biases post-calibration.
• Incorporated recent advancements in cosmological inference from the joint DES Year 3 and KiDS-1000 cosmic shear analysis.
• Employed a redshift-independent NLA model with a narrow flat prior on the IA amplitude $A_{\rm IA}$, and tested alternative models including broad prior and redshift-evolving $A_{\rm IA}$.
• Propagated systematic uncertainties into the covariance matrix, treating them as residual biases after calibration.

Experimental results

Research questions

• RQ1 How do improved galaxy shape measurements and shear calibration affect cosmological constraints in the KiDS-1000 cosmic shear survey?
• RQ2 To what extent do systematic uncertainties from image simulation realism and shape measurement algorithm sensitivity contribute to the $S_8$ error budget?
• RQ3 How do different intrinsic alignment (IA) modeling assumptions affect the derived $S_8$ constraints?
• RQ4 What is the level of tension between the KiDS-1000 $S_8$ measurement and Planck CMB constraints under improved systematics control?
• RQ5 How consistent are the results with previous KiDS and DES cosmic shear analyses when using enhanced calibration and measurement techniques?

Key findings

• The refined analysis yields $S_8 = 0.776^{+0.029+0.002}_{-0.027-0.003}$, with the second uncertainty set reflecting systematic errors in shear calibration.
• The $S_8$ constraint shows a $\sim2.3\sigma$ tension with Planck CMB results, consistent with previous weak lensing surveys.
• Systematic uncertainties from shear calibration contribute approximately 8% to the total uncertainty on $S_8$.
• The results are robust across different intrinsic alignment (IA) modeling assumptions, including redshift-independent and redshift-evolving IA models.
• The joint analysis with DES Year 3 data shows $S_8 = 0.790^{+0.018}_{-0.014}$, indicating a reduced tension with Planck, but KiDS-1000 alone still shows significant discrepancy.
• The study confirms that enhanced image simulations and improved shape measurement algorithms effectively reduce systematic biases in cosmic shear cosmology.