KiDS-450 + 2dFLenS: Cosmological parameter constraints from weak gravitational lensing tomography and overlapping redshift-space galaxy clustering (1707.06627v2)

Abstract: We perform a combined analysis of cosmic shear tomography, galaxy-galaxy lensing tomography, and redshift-space multipole power spectra (monopole and quadrupole) using 450 deg$^2$ of imaging data by the Kilo Degree Survey (KiDS) overlapping with two spectroscopic surveys: the 2-degree Field Lensing Survey (2dFLenS) and the Baryon Oscillation Spectroscopic Survey (BOSS). We restrict the galaxy-galaxy lensing and multipole power spectrum measurements to the overlapping regions with KiDS, and self-consistently compute the full covariance between the different observables using a large suite of $N$-body simulations. We methodically analyze different combinations of the observables, finding that galaxy-galaxy lensing measurements are particularly useful in improving the constraint on the intrinsic alignment amplitude (by 30%, positive at $3.5\sigma$ in the fiducial data analysis), while the multipole power spectra are useful in tightening the constraints along the lensing degeneracy direction (e.g. factor of two stronger matter density constraint in the fiducial analysis). The fully combined constraint on $S_8 \equiv \sigma_8 \sqrt{\Omega_{\rm m}/0.3} = 0.742 \pm 0.035$, which is an improvement by 20% compared to KiDS alone, corresponds to a $2.6\sigma$ discordance with Planck, and is not significantly affected by fitting to a more conservative set of scales. Given the tightening of the parameter space, we are unable to resolve the discordance with an extended cosmology that is simultaneously favored in a model selection sense, including the sum of neutrino masses, curvature, evolving dark energy, and modified gravity. The complementarity of our observables allows for constraints on modified gravity degrees of freedom that are not simultaneously bounded with either probe alone, and up to a factor of three improvement in the $S_8$ constraint in the extended cosmology compared to KiDS alone.

• The paper presents a novel integration of cosmic shear, galaxy-galaxy lensing, and redshift-space clustering to deliver tighter constraints on cosmological parameters.
• It employs 930 N-body simulations to develop a robust covariance matrix that improves precision in estimating intrinsic alignments and matter density.
• The analysis identifies a 2.6σ discordance with Planck CMB data, reinforcing the ΛCDM model while highlighting potential systematic effects for future research.

Summary of "KiDS-450 + 2dFLenS: Cosmological parameter constraints from weak gravitational lensing tomography and overlapping redshift-space galaxy clustering"

The paper, "KiDS-450 + 2dFLenS: Cosmological parameter constraints from weak gravitational lensing tomography and overlapping redshift-space galaxy clustering" by Joudaki et al., presents a meticulous analysis combining cosmic shear, galaxy-galaxy lensing, and redshift-space multipole power spectra. Utilizing datasets from the Kilo Degree Survey (KiDS-450) that overlap with the spectroscopic 2-degree Field Lensing Survey (2dFLenS) and the Baryon Oscillation Spectroscopic Survey (BOSS), the paper aims to set constraints on cosmological parameters by leveraging these complementary observational datasets.

Methodological Approach

The authors conduct a comprehensive analysis involving three main cosmological observables: the tomographic two-point shear correlation functions (cosmic shear), the tomographic galaxy-galaxy lensing angular cross-correlation, and the redshift-space multipole power spectra (including monopole and quadrupole). The integration of these observables allows for tight cosmological parameter constraints. Notably, the paper incorporates a self-consistent covariance matrix calculated using 930 N-body simulations, ensuring robust treatment of statistical errors.

The authors improve constraints on intrinsic alignment by 30% and achieve significant enhancements in measurements along the lensing degeneracy direction, doubling the precision on matter density constraints, amongst others. The Bayesian Monte Carlo Markov chain (MCMC) methodology used for cosmological parameter extraction is extended to include factors such as galaxy bias, pairwise velocity dispersion, and more, allowing them to handle systematic uncertainties methodically.

Key Results and Findings

A substantial result from the paper is the observed discordance (not resolved through extended cosmological models tested in this research) between their observations and Planck CMB measurements, with deviations in terms of intrinsic parameters like the sum of neutrino masses, curvature, and evolving dark energy metrics. Specifically, the discordance quantified was approximately 2.6σ for certain parameter combinations, signaling the necessity for further model exploration or systematic investigations.

The authors effectively rule out large modifications to General Relativity by constraining deviations using an effective degree of freedom model. According to their findings, none of the deviations considered provide a compelling alternative to the ΛCDM model, further solidifying the standard model's current standing.

Implications for Future Research

This work emphasizes the growing importance of combining different cosmological probes to break degeneracies inherent to individual measurements. It sets the stage for future high-precision measurements and analyses that incorporate ever larger survey datasets, such as those expected from upcoming Euclid and LSST observations. Importantly, the paper also makes available the pipeline used, paving the way for reproducibility and further community-wide investigation into the noted discrepancies.

Future research directions stemming from this paper may involve dissecting the causes of the noted discordances between various datasets, particularly exploring systematic biases or evolutionary components which might not be captured by the current models. Extensions to this analysis could include refined treatment of baryonic effects on smaller scales or further probing into the parameters impacting intrinsic alignments.

Conclusion

This paper provides a robust and comprehensive approach to cosmological parameter estimation using overlapping survey data. By advancing methodologies and tools to treat systematic uncertainties, the research undoubtedly contributes valuable insights towards understanding the universe's large-scale structure dynamics and stresses the importance of multi-faceted data integrations in cosmology. As ongoing and future surveys expand the horizon of observational cosmology, the pioneering techniques and findings of this paper will likely remain highly influential.