DES Y3 + KiDS-1000: Consistent cosmology combining cosmic shear surveys

Abstract: We present a joint cosmic shear analysis of the Dark Energy Survey (DES Y3) and the Kilo-Degree Survey (KiDS-1000) in a collaborative effort between the two survey teams. We find consistent cosmological parameter constraints between DES Y3 and KiDS-1000 which, when combined in a joint-survey analysis, constrain the parameter $S_8 = \sigma_8 \sqrt{\Omega_{\rm m}/0.3}$ with a mean value of $0.790^{+0.018}_{-0.014}$. The mean marginal is lower than the maximum a posteriori estimate, $S_8=0.801$, owing to skewness in the marginal distribution and projection effects in the multi-dimensional parameter space. Our results are consistent with $S_8$ constraints from observations of the cosmic microwave background by Planck, with agreement at the $1.7\sigma$ level. We use a Hybrid analysis pipeline, defined from a mock survey study quantifying the impact of the different analysis choices originally adopted by each survey team. We review intrinsic alignment models, baryon feedback mitigation strategies, priors, samplers and models of the non-linear matter power spectrum.

Overview of "DES Y3 + KiDS-1000: Consistent Cosmology combining cosmic shear surveys"

This paper presents a joint cosmic shear analysis leveraging data from the Dark Energy Survey (DES Y3) and the Kilo-Degree Survey (KiDS-1000). The primary aim is to provide consistent cosmological parameter constraints by combining these two significant cosmic shear surveys. The combined analysis results in a highly precise measurement of the parameter $S_8 = \sigma_8 \sqrt{\Omega_m/0.3}$, with a mean value of $0.790^{+0.018}_{-0.014}$. The findings are in agreement with cosmic shear observations derived from the cosmic microwave background as observed by the Planck satellite at a $1.7\sigma$ level.

Methodology

The authors employ a "Hybrid analysis pipeline" developed from comprehensive studies involving mock surveys to assess the impact of different analysis choices initially adopted by the DES and KiDS teams. This includes evaluating intrinsic alignment models, baryon feedback mitigation strategies, priors, sampling algorithms, and models for non-linear matter power spectrum. This hybrid approach allows for a unified analysis that marries the strengths of both survey methodologies, aiming to address and minimize biases that could exist due to methodological divergences.

Key Findings

• Cosmological Parameter Constraints: The joint analysis provides constraints that are consistent with individual survey parameters, cross-validating each other's results and further aligning with theoretical expectations from Planck's CMB observations.
• Impact of Survey Methodologies: The unification into a Hybrid pipeline demonstrated the importance of choosing appropriate intrinsic alignment models and accounting for baryon feedback, highlighting how these factors can significantly affect outcomes.
• Projection Effects: The choice of sampling parameters demonstrated a bias when unconstrained parameters are misaligned with underlying truths, suggesting careful consideration is required to avoid erroneous conclusions.
• Bias Mitigation: Strategies combining scale cuts and baryon feedback marginalization proved effective in countering potential biases from scale-dependent baryonic effects.

Implications and Future Directions

The study showcases that collaborative and unified analyses significantly enhance the robustness and precision of cosmological parameter constraints. Furthermore, the results emphasize the necessity of developing strategies to mitigate model biases—particularly those stemming from intrinsic alignment and baryon feedback modeling. Going forward, the integration of next-generation cosmic shear surveys such as Euclid, Nancy Grace Roman Space Telescope, and the Vera C. Rubin Observatory will benefit from the insights and methodologies developed in this joint study. Additionally, increasing precision demands will require a deeper understanding and possibly a revision or extension of intrinsic alignment models to account for more complex astrophysical phenomena.

Conclusion

This paper highlights the capability and necessity of cross-survey analyses in establishing consistent cosmological models. It serves as a benchmark for future collaborative projects, providing a robust methodological framework that assures precision in cosmological findings. Future efforts may be focused on expanding this model to other large-scale surveys, enhancing the understanding of cosmic evolution and dark energy consistency across the universe.