dynamical relaxationA process occurring within a cluster of galaxies in which the collective gravitational potential acts on individual constituent galaxies. During the early stages of cluster formation the energy of a galaxy fluctuates very rapidly as the cluster undergoes violent relaxation to settle into a constant potential in virial equilibrium. This process is modeled using n-body simulations that assume that gravitational interactions between individual galaxies are ignored in favor of modeling their response to the total mass. Once the cluster has collapsed, relaxation continues through two-body gravitational interactions. The galaxies lose orbital energy through the process and slow down due to this dynamical friction. This is particularly important for the most massive galaxies that gradually settle to the cluster center and may merge to form the massive dominant galaxy, often a cD galaxy. Most clusters are in a process of segregation of slow-moving massive galaxies at low cluster radii and lighter galaxies further out.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006