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Self-similar energetics in large clusters of galaxies

Massive galaxy clusters are filled with a hot, turbulent and magnetized intra-cluster medium. Still forming under the action of gravitational instability, they grow in mass by accretion of supersonic flows. These flows partially dissipate into heat through a complex network of large-scale shocks [1], while residual transonic flows create giant turbulent eddies and cascades [2,3]. Turbulence heats the intra-cluster medium [4] and also amplifies magnetic energy by way of dynamo action [5-8]. However, the pattern regulating the transformation of gravitational energy into kinetic, thermal, turbulent and magnetic energies remains unknown. Here we report that the energy components of the intra-cluster medium are ordered according to a permanent hierarchy, in which the ratio of thermal to turbulent to magnetic energy densities remains virtually unaltered throughout the cluster's history, despite evolution of each individual component and the drive towards equipartition of the turbulent dynamo. This result revolves around the approximately constant efficiency of turbulence generation from the gravitational energy that is freed during mass accretion, revealed by our computational model of cosmological structure formation [3,9]. The permanent character of this hierarchy reflects yet another type of self-similarity in cosmology [10-13], while its structure, consistent with current data [14-18], encodes information about the efficiency of turbulent heating and dynamo action.