Robust Weak-lensing Mass Calibration of Planck Galaxy Clusters (1402.2670v2)

Abstract: In light of the tension in cosmological constraints reported by the Planck team between their SZ-selected cluster counts and Cosmic Microwave Background (CMB) temperature anisotropies, we compare the Planck cluster mass estimates with robust, weak-lensing mass measurements from the Weighing the Giants (WtG) project. For the 22 clusters in common between the Planck cosmology sample and WtG, we find an overall mass ratio of $\left< M_{Planck}/M_{\rm WtG} \right> = 0.688 \pm 0.072$. Extending the sample to clusters not used in the Planck cosmology analysis yields a consistent value of $\left< M_{Planck}/M_{\rm WtG} \right> = 0.698 \pm 0.062$ from 38 clusters in common. Identifying the weak-lensing masses as proxies for the true cluster mass (on average), these ratios are $\sim 1.6\sigma$ lower than the default mass bias of 0.8 assumed in the Planck cluster analysis. Adopting the WtG weak-lensing-based mass calibration would substantially reduce the tension found between the Planck cluster count cosmology results and those from CMB temperature anisotropies, thereby dispensing of the need for "new physics" such as uncomfortably large neutrino masses (in the context of the measured Planck temperature anisotropies and other data). We also find modest evidence (at 95 per cent confidence) for a mass dependence of the calibration ratio and discuss its potential origin in light of systematic uncertainties in the temperature calibration of the X-ray measurements used to calibrate the Planck cluster masses. Our results exemplify the critical role that robust absolute mass calibration plays in cluster cosmology, and the invaluable role of accurate weak-lensing mass measurements in this regard.

• The paper recalibrates Planck cluster masses using robust weak-lensing measurements from the Weighing the Giants project.
• It finds a mass ratio of about 0.69, indicating that Planck’s default bias underestimates true cluster masses by 5–25%.
• The recalibration lessens the tension between SZ cluster counts and CMB observations, emphasizing the need for multi-wavelength data in cosmological models.

Robust Weak-Lensing Mass Calibration of Planck Galaxy Clusters

The paper, "Robust weak-lensing mass calibration of Planck galaxy clusters" by Anja von der Linden et al., presents a rigorous analysis of cluster mass estimates, specifically focusing on the tension between the Planck SZ-selected cluster counts and Cosmic Microwave Background (CMB) temperature anisotropies. The authors utilize weak-lensing measurements from the "Weighing the Giants" (WtG) project to provide a recalibration of these masses, thereby shedding light on possible biases in the Planck team's analysis.

Methodology and Findings

The paper employs robust, unbiased weak-lensing mass measurements, identified as proxies for true cluster masses, to recalibrate the Planck results. The core comparison involves 22 clusters within both the Planck cosmological sample and WtG dataset, yielding a mass ratio of $$\left< M_{Planck}/M_{\rm WtG} \right> = 0.688 \pm 0.072$$. Extending the analysis to 38 clusters corroborates these results with a consistent value of $$\left< M_{Planck}/M_{\rm WtG} \right> = 0.698 \pm 0.062$$. These findings indicate that the default bias factor $(1-b) = 0.8$ assumed by Planck underestimates the true cluster masses by approximately 5–25%.

Implications

The WtG-based calibration could significantly reduce the disparity seen between Planck's SZ cluster counts and CMB results, mitigating the necessity for hypotheses involving massive neutrinos. The recalibration supports the argument that the existing tension is likely due to inadequate mass calibration rather than new physics scenarios.

Moreover, the paper reveals modest evidence for a mass-dependent bias in the calibration ratio, potentially stemming from systematic uncertainties in X-ray temperature calibration used by Planck. This insight underscores the critical role astrophysical observations play, particularly emphasizing the precision offered by weak-lensing in establishing mass calibrations independent of baryonic models.

Future Prospect and Impact

The results advocate for the integration of weak-lensing measurements as a complementary tool in cluster cosmology, enhancing the accuracy of mass determinations that are pivotal for cosmological parameter estimation. The collaboration of multi-wavelength data—spanning X-ray, SZ, and lensing observations—demonstrates the importance of combining methodologies to address systematic biases.

The paper also alludes to ongoing efforts, primarily through further refinement of cosmological constraint strategies utilizing the WtG dataset, promising advancements in understanding large-scale cosmic evolution and structure formation.

This paper exemplifies the intricate connection between observational techniques and cosmological inference, advocating for continued cross-validation of astrophysical data to bolster cosmological models and predictions.