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.
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