Galaxy clusters, enormous cosmic structures held together by gravity, are excellent natural laboratories, particularly suitable for determining the properties of dark matter. It was thanks to the analysis of the dynamics of a cluster of galaxies that the first evidence of dark matter was obtained, more than 90 years ago (Zwicky 1933). To date, the most convincing proof of the existence of dark matter is due to the study of another cluster, the “Bullet Cluster” (Clowe et al. 2006). Thanks to observations with increasingly better instruments, today the study of the dynamics of clusters allows us to place constraints not only on the existence, but also on the nature of dark matter.
In particular, an international collaboration directed by Piero Rosati of the University of Ferrara, in which two researchers from the OATs and the University of Trieste also participate, has recently collected very high quality spectroscopic data for some galaxy clusters with the MUSE instrument of ESO's Very Large Telescope. These data have allowed us to determine with great precision the kinematics of the galaxies within these clusters as well as the stellar kinematics within the dominant galaxies at their centers (BCG, Brightest Cluster Galaxies).
By combining the stellar kinematics of the BCGs with that of the other galaxies of two clusters, it was possible to determine the radial mass distribution profile of dark matter and compare it with that predicted by cosmological models of cold dark matter (CDM), the so-called NFW profile , from the initials of its authors (Navarro, Frenk & White 1996, 1997). Previous studies had concluded that the mass profile of dark matter in galaxy clusters was not in agreement with the theoretical CDM model (Sand et al. 2004, Newman et al. 2013). The new analyses, published in two articles in prestigious scientific journals (Sartoris et al. 2020, Biviano et al. 2023), lead instead to the opposite conclusion: there is excellent agreement between the observed and theoretical profiles of CDM. The difference between the new results and previous ones is partly due to the use of new kinematic data from many hundreds of cluster galaxies, and partly to the extraordinary quality of the data obtained from ESO's MUSE spectrograph.
