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Cosmological parameters from measurements of the sparsity of galaxy clusters

8 March 2021

A team of astrophysicists from the Paris Observatory and INAF-Bologna has performed an estimation of the cosmological parameters using a new method based on measurements of the level of sparsity of galaxy clusters. The researchers were able to determine the cosmic matter density, its level of clustering and the rate of cosmic expansion, which contrary to estimates based on cluster number counts, are in good agreement with the results of the analyses of the cosmic microwave background from the Planck satellite.

Galaxy clusters are among the largest and most massive structures in the universe. By comprising several tens or even thousands of galaxies within vast dark matter halos, these are the ultimate result of the hierarchical process of cosmic structure formation. Triggered by the gravitational infall of matter, the baryonic gas collapses within dark matter halos to form the stars and galaxies we observe today. Throughout the cosmic evolution, the subsequent mergers of these halos lead to the formation of the largest and most massive ones which make the bulk of galaxy clusters.

Observations of these structures can provide information on the global properties of the universe, such as the cosmic matter density or the level of clustering of matter. During the past years the realization of numerous galaxy cluster surveys has enabled estimations of these parameters from measurements of cluster number counts. Surprisingly, the majority of these studies has found values that are slightly smaller than those inferred from the analysis of the maps of the Cosmic Microwave Background (CMB) from the Planck satellite. However, several systematic effects inherent to the cluster count measurements may be at the origin of this discrepancy.

A team of astrophysicists from the Observatory of Paris and the National Institute of Astrophysics of Bologna has realized for the first time a cosmological analysis based on a novel methodology that allows to estimate the cosmological parameters from measurements of the internal mass distribution of clusters as estimated by a proxy of their “sparsity”. Using measurements of the sparsity from a sample of 317 clusters, the researchers have estimated the cosmic matter density and the amplitude of the clustering of matter on the 12 Mpc scale with a precision that is comparable to that of other standard probes. The results are in agreement with those from the CMB (see attached figure). It is worth noticing that this method also allows to measure the cosmic expansion rate, which is also found to be in agreement with the results of the Planck data analysis and smaller than the results of distance measurements in the local universe from observations of the Hubble Space Telescope.

Figure: Likelihood contours delimiting the combinations of cosmological parameter values (cosmic matter density Omegam, and amplitude of density fluctuations sigma8) in agreement with the observational data respectively with a probability of 68% (internal contours) and 95% (external contours). The results of the analysis of the sparsity of clusters (in combination with the gas fraction of the clusters and the BAO) correspond to the black solid line contours, those obtained from the Planck-SZ cluster counts (in combination with the constraints of the Big-Bang Nucleosynthesis and BAO) correspond to the shaded light and dark blue regions, those from the analysis of cosmic microwave background from Planck are given by the red and yellow shaded regions, and those resulting from the clustering of matter measured by the shear gravitational lensing of galaxies are given by light and dark beige regions.

The use of the cluster sparsity opens new ways of studying the properties of galaxy clusters. In the future, the analysis of very large cluster samples, such as those expected from the Euclid satellite, will allow an even more precise estimate of the cosmological parameters.

Reference
P.S. Corasaniti, M. Sereno, S. Ettori, « Cosmological constraints from galaxy cluster sparsity, cluster gas mass fraction and baryon acoustic oscillations data »,
Astrophys. J., 2021, in press