Weighing the Giants - I. Weak-lensing masses for 51 massive galaxy clusters: project overview, data analysis methods and cluster images

Abstract: This is the first in a series of papers in which we measure accurate weak-lensing masses for 51 of the most X-ray luminous galaxy clusters known at redshifts 0.15<z<0.7, in order to calibrate X-ray and other mass proxies for cosmological cluster experiments. The primary aim is to improve the absolute mass calibration of cluster observables, currently the dominant systematic uncertainty for cluster count experiments. Key elements of this work are the rigorous quantification of systematic uncertainties, high-quality data reduction and photometric calibration, and the "blind" nature of the analysis to avoid confirmation bias. Our target clusters are drawn from RASS X-ray catalogs, and provide a versatile calibration sample for many aspects of cluster cosmology. We have acquired wide-field, high-quality imaging using the Subaru and CFHT telescopes for all 51 clusters, in at least three bands per cluster. For a subset of 27 clusters, we have data in at least five bands, allowing accurate photo-z estimates of lensed galaxies. In this paper, we describe the cluster sample and observations, and detail the processing of the SuprimeCam data to yield high-quality images suitable for robust weak-lensing shape measurements and precision photometry. For each cluster, we present wide-field color optical images and maps of the weak-lensing mass distribution, the optical light distribution, and the X-ray emission, providing insights into the large-scale structure in which the clusters are embedded. We measure the offsets between X-ray centroids and Brightest Cluster Galaxies in the clusters, finding these to be small in general, with a median of 20kpc. For offsets <100kpc, weak-lensing mass measurements centered on the BCGs agree well with values determined relative to the X-ray centroids; miscentering is therefore not a significant source of systematic uncertainty for our mass measurements. [abridged]

Weighing the Giants: Weak-Lensing Masses for Massive Galaxy Clusters

The paper "Weighing the Giants" by Anja von der Linden et al. investigates weak-lensing masses for 51 massive galaxy clusters, with the primary purpose of calibrating X-ray and other mass proxies commonly used in cosmological cluster experiments. The central objective of this study is to enhance the mass calibration of cluster observables, which currently serves as the most significant source of systematic uncertainty in cluster count experiments. This research represents the first part of a series of studies aiming to provide accurate weak-lensing metrics that support more precise cosmological parameter determinations.

In this study, the authors utilize weak lensing to obtain robust mass estimates for galaxy clusters, acknowledging the inherent challenges due to intrinsic scatter and systematic biases in lens measurements. Their methodological approach emphasizes high-quality data reduction and photometric calibration, underpinned by "blind" analyses designed to minimize confirmation bias. The researchers employ data from the ROSAT All-Sky Survey, Subaru, and CFHT telescopes, ensuring a wide-field imaging strategy in multiple optical bands.

The sample clusters, selected for their X-ray properties, span redshifts $0.15 < z < 0.7$, ensuring a comprehensive dataset that supports calibration over a range of redshifts pertinent to current cosmological inquiries. The study discusses the techniques employed for the cluster lensing analysis, such as the KSB method for shear measurements, curse of large-scale systematic effects, and photometric redshift evaluations. Choices regarding data processing, including adjustments for scattered light and correlated noise, are meticulously documented to improve throughput in shear signal interpretation.

A key aspect of this paper is the comparison of weak-lensing centers to alternative measures, such as X-ray centroids and Brightest Cluster Galaxies (BCGs). Results suggest that miscentering effects are not significant when X-ray centroids are used, allowing the study to effectively minimize potential biases in mass estimates. The authors also present the aperture mass maps, maximizing the signal of real halos and giving insight into the large-scale structure around clusters.

Furthermore, this paper details the construction of cluster maps, employing virial scales for detailed mass, light, and X-ray emission contours. This multi-faceted approach provides a holistic view and verification of cluster alignment with observed light and weak-lensing signatures. The analysis reveals typical offsets between observed features and real mass concentrations, which are consistent with theoretical expectations derived from simulations.

Importantly, the paper introduces forthcoming analyses in subsequent parts of the series, where the masses derived in this study will be integrated into a broader framework to establish both cluster scaling relations and meaningful cosmological parameter constraints. This integrative approach allows the alignment of theoretical and observational methodologies toward a shared cosmological understanding.

Finally, the paper acknowledges the limitations of current methodologies and provides clear paths for future work. By establishing an unprecedented dataset with rigorous handling of systematic uncertainties, this study lays the groundwork for improved calibration of mass proxies, which is critical for the efficacy of upcoming large-scale surveys such as the Dark Energy Survey (DES) and eROSITA.

In conclusion, von der Linden et al.'s work contributes significantly to the field of cluster cosmology by addressing fundamental challenges in mass calibration. This research underscores the importance of weak lensing as a powerful tool for unlocking precise cosmological information from clusters, paving the way for advancements in both theoretical modeling and observational astrophysics.