Weighing the Giants - III. Methods and Measurements of Accurate Galaxy Cluster Weak-Lensing Masses (1208.0605v2)

Abstract: We report weak-lensing masses for 51 of the most X-ray luminous galaxy clusters known. This cluster sample, introduced earlier in this series of papers, spans redshifts 0.15 < z_cl < 0.7, and is well suited to calibrate mass proxies for current cluster cosmology experiments. Cluster masses are measured with a standard color-cut' lensing method from three-filter photometry of each field. Additionally, for 27 cluster fields with at least five-filter photometry, we measure high-accuracy masses using a new method that exploits all information available in the photometric redshift posterior probability distributions of individual galaxies. Using simulations based on the COSMOS-30 catalog, we demonstrate control of systematic biases in the mean mass of the sample with this method, from photometric redshift biases and associated uncertainties, to better than 3%. In contrast, we show that the use of single-point estimators in place of the full photometric redshift posterior distributions can lead to significant redshift-dependent biases on cluster masses. The performance of our new photometric redshift-based method allows us to calibratecolor-cut` masses for all 51 clusters in the present sample to a total systematic uncertainty of ~7% on the mean mass, a level sufficient to significantly improve current cosmology constraints from galaxy clusters. Our results bode well for future cosmological studies of clusters, potentially reducing the need for exhaustive spectroscopic calibration surveys as compared to other techniques, when deep, multi-filter optical and near-IR imaging surveys are coupled with robust photometric redshift methods.

• The paper demonstrates that applying the P(z) method yields mean mass measurements with systematic uncertainties under 2%.
• It contrasts traditional color-cut and advanced photometric redshift techniques to calibrate cluster mass proxies effectively.
• The findings minimize the need for extensive spectroscopic surveys, enhancing prospects for future DES and LSST studies.

An Analytical Perspective on Weak Lensing Mass Measurements in X-ray Luminous Galaxy Clusters

This paper presents detailed weak-lensing mass measurements for 51 of the most X-ray luminous galaxy clusters, spanning redshifts from 0.15 to 0.7. The authors utilize two distinct methodologies: a traditional "color-cut" method and an advanced approach based on photometric redshift probability distributions, denoted as the "$P(z)$" method. Both methods serve to calibrate mass proxies for ongoing cluster cosmology studies, with the $P(z)$ method offering a significant advantage in controlling systematic uncertainties, reportedly below 2%. This constitutes a pivotal step towards reducing reliance on exhaustive spectroscopic calibration surveys.

Color-Cut vs. Photometric Redshift Methods

The color-cut method relies on photometry in at least three filters and estimates the redshift distribution from a reference deep field. However, this approach poses challenges in systematic uncertainty quantification, attributed to cosmic variance and contamination corrections. The thorough cross-calibration conducted in this paper establishes a systematic uncertainty of approximately 7% on the mean mass when integrated with the $P(z)$ method results.

The $P(z)$ method, utilizing robust photometric redshift posterior probabilities, has shown to yield mean mass measurements accurate to better than 2%. Notably, its implementation does not require the numerous spectroscopic redshifts traditionally needed for calibration, making it a promising alternative for future surveys, such as DES and LSST, which will offer extensive multi-filter datasets. This approach is significantly less susceptible to redshift-dependent mass biases, a critical factor for high redshift clusters.

Systematic Uncertainties and Methodological Rigor

The authors meticulously address systematic biases in shear measurements, instrumental inconsistencies, and the mass model assumptions intrinsic to lensing analyses. The evaluated systematic uncertainties from these sources collectively contribute a notable 4% to the total uncertainty. The analysis reveals that biases from the assumed NFW halo profiles and concentration relations remain consistent with simulation-based expectations.

Comparison to Literature

The weak-lensing masses reported are in general alignment with contemporary literature, with notable exceptions. The comparison with prior studies, such as those by Okabe et al. and Hoekstra et al., reveals systematic discrepancies, often tied to differing analysis depth and shear calibration techniques. These deviations underscore the need for meticulous calibration in lensing studies.

Implications and Future Prospects

The precision achieved in mean mass measurements, underscored by rigorous simulations and methodical cross-comparisons, positions this paper as a cornerstone for accurate cluster mass calibration in cosmological contexts. By reducing the dependence on spectroscopic data, the $P(z)$ method exemplifies a methodological shift that can enhance the efficacy of future cluster surveys. This work advocates for continuing advancements in simulation realism and photometric calibration to further minimize systematic biases. Thus, it highlights the $P(z)$ method's potential to improve cosmological constraints derived from cluster surveys by delivering accurate mass measurements essential for characterizing the dark matter distribution in the universe.