Using a sample of 54 galaxy clusters with data collected by the WINGS collaboration (Fasano et al. 2006), a new study conducted by a researcher from OATs has determined with great precision that the PSD profile of these clusters is very close to a specific power law, predicted by various theoretical studies and numerical simulations (e.g. Taylor & Navarro 2001). The theoretical interpretation of this law is that of a state resulting from a dynamic relaxation process (described by Lynden-Bell already in 1967) which occurs on a time scale of 1-2 billion years, equivalent to about 10-20% of the average age of the clusters.
The new study showed that the same PSD profile is obtained using both elliptical and spiral galaxies as tarcers of the cluster gravitational potential. This result indicates that it is necessary to think of two different dynamic relaxation mechanisms leading to the same PSD profile. In fact, the Lynden-Bell process cannot work for the galaxies that we currently identify as spirals, because these cannot maintain their characteristic morphology for a long time inside the clusters, due to various evolutionary processes, and must therefore have entered the clusters no more than 3 billion years ago (as suggested by Cava et al. 2017 among others). It is therefore necessary to invoke another, slower and smoother, dynamic relaxation process, which allows the newly arrived galaxies in the clusters to gradually modify their distribution until this traces the same PSD profile traced by their older elliptical co-inmates.
The new study concludes with a consideration on lenticular or S0 galaxies, which unlike ellipticals and spirals, seem to trace the PSD profile in a different way from what is predicted by numerical and theoretical studies. The S0 galaxies would be a mixture of two populations of galaxies, one originating in the early stages of clusters, such as ellipticals, and one evolved from spiral galaxies in more recent times.
