Planck 2015 results. XXIV. Cosmology from Sunyaev-Zeldovich cluster counts (1502.01597v3)

Abstract: We present cluster counts and corresponding cosmological constraints from the Planck full mission data set. Our catalogue consists of 439 clusters detected via their Sunyaev-Zeldovich (SZ) signal down to a signal-to-noise ratio of 6, and is more than a factor of 2 larger than the 2013 Planck cluster cosmology sample. The counts are consistent with those from 2013 and yield compatible constraints under the same modelling assumptions. Taking advantage of the larger catalogue, we extend our analysis to the two-dimensional distribution in redshift and signal-to-noise. We use mass estimates from two recent studies of gravitational lensing of background galaxies by Planck clusters to provide priors on the hydrostatic bias parameter, $(1-b)$. In addition, we use lensing of cosmic microwave background (CMB) temperature fluctuations by Planck clusters as an independent constraint on this parameter. These various calibrations imply constraints on the present-day amplitude of matter fluctuations in varying degrees of tension with those from the Planck analysis of primary fluctuations in the CMB; for the lowest estimated values of $(1-b)$ the tension is mild, only a little over one standard deviation, while it remains substantial ($3.7\,\sigma$) for the largest estimated value. We also examine constraints on extensions to the base flat $\Lambda$CDM model by combining the cluster and CMB constraints. The combination appears to favour non-minimal neutrino masses, but this possibility does little to relieve the overall tension because it simultaneously lowers the implied value of the Hubble parameter, thereby exacerbating the discrepancy with most current astrophysical estimates. Improving the precision of cluster mass calibrations from the current 10%-level to 1% would significantly strengthen these combined analyses and provide a stringent test of the base $\Lambda$CDM model.

• The paper expands the SZ cluster sample to 439 galaxies, doubling previous counts and providing robust cosmological constraints.
• It utilizes a two-dimensional redshift and SNR distribution with gravitational lensing to refine mass estimates and hydrostatic bias parameters.
• The findings reveal significant tension with CMB-derived parameters, highlighting the need for improved cluster mass calibrations.

Cosmology from Sunyaev-Zeldovich Cluster Counts: Insights from Planck 2015 Results

The Planck 2015 results represent a significant contribution to the field of cosmology through the analysis of galaxy clusters detected via the Sunyaev-Zeldovich (SZ) effect. This paper provides a comprehensive analysis of the cluster counts from the full mission data set of the Planck satellite, significantly expanding upon the earlier 2013 results.

Cluster Counts and Sample Size

The paper reports the detection of 439 galaxy clusters with a signal-to-noise ratio (SNR) greater than 6, surpassing the 2013 sample size by more than a factor of two. This increase in sample size provides enhanced statistical power, allowing for more robust constraints on cosmological parameters. The analysis extends to a two-dimensional distribution in redshift and SNR, which provides a more detailed understanding of the cluster counts compared to the previous one-dimensional approach.

Cosmological Implications

The abundance and distribution of galaxy clusters are sensitive probes of cosmological parameters, particularly the matter density ($\Omega_m$) and the normalization of the matter power spectrum ($\sigma_8$). The paper employs mass estimates from gravitational lensing to constrain the hydrostatic bias parameter, $(1-b)$. The constraints on this parameter play a critical role in deriving cosmological insights, as it accounts for potential underestimation of cluster masses due to non-thermal pressure support.

Constraints and Tension with CMB Data

The analysis indicates a mild to substantial tension between the cluster-derived cosmological constraints and those from the primary fluctuations of the Cosmic Microwave Background (CMB) measured by Planck. This tension is particularly notable for the largest estimated values of $(1-b)$, reaching up to $3.7\,\sigma$. The paper also explores extensions to the base flat $\Lambda$CDM model, such as non-minimal neutrino masses, which, while intriguing, do little to alleviate the tension due to their effects on the Hubble parameter.

Future Directions

The authors highlight the critical need to improve the precision of cluster mass calibrations. The current uncertainty at the 10\% level needs to be reduced to 1\% or less to leverage the full potential of cluster counts in testing the $\Lambda$CDM model. Achieving such precision would provide a more stringent test of the model and could significantly enhance the constraints on possible extensions to it.

Conclusion

The Planck 2015 analysis of SZ-selected galaxy clusters advances our understanding of cosmology by improving sample size and methodological approaches. However, the noted tensions between cluster counts and CMB data underscore the necessity for improved mass calibration techniques and potentially point towards the need for new physics beyond the standard cosmological model. This work lays the groundwork for future research that could refine our understanding of the universe's composition and evolution.