Self-similarity of Dynamo Action in the Largest Cosmic Structures

Massive galaxy clusters (GC) are filled with a hot, turbulent and magnetised intra-cluster medium (ICM). 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 shocks1, while residual transonic flows create giant turbulent eddies and cascade2, 3. Turbulence heats the ICM4 and also amplifies magnetic energy by way of dynamo action5–8. However, fundamental properties of the pattern whereby gravitational energy turns kinetic, thermal, turbulent and magnetic remain unknown. Here we find that the energy components of the ICM are ordered according to a permanent hierarchy, in which the ratios of thermal to turbulent to magnetic energy densities remain virtually unaltered throughout the ICM history despite evolution of each individual component and the drive towards equipartition of turbulent dynamo. Our results are based on a state-of-the-art, fully cosmological computational model of ICM turbulence3, 9, revealing that an approximately constant efficiency of turbulence generation from gravitational energy that is freed during mass accretion. The permanent character of this hierarchy reflects a new aspect of self-