Andrey Beresnyak
Naval Research Lab
Nonlinear dynamo in the intracluster medium
Massive galaxy clusters are filled with a hot, turbulent and magnetized intra-cluster medium (ICM). Still forming under the action of gravity, they continually grow in mass even today. Turbulence heats the ICM and also amplifies magnetic field by way of dynamo action. This magnetic field was observed by Faraday rotation of background sources to have a magnitude of around a microGauss and the correlation scale of tens of kiloparsecs - the largest correlation scales so far observed in the Universe. The large scale of this field sparked a hope that they could be somehow connected to the primordial magnetic field in the early Universe - potentially shedding light on details of quantum field theory, such as phase transitions. A number of brute-force attempts to simulate cluster dynamo in cosmological context with MHD codes resulted in an optimistic view that the magnitude of cluster magnetic field at present time indeed sensitively depend on primordial field. In this talk I will explain in detail why this is likely to be erroneous. For this purpose I will overview the modern understanding of kinematic and nonlinear turbulent fluctuation dynamo - a kind of dynamo that operates in galaxy cluster. Using this knowledge we come to, perhaps counter-intuitive, conclusion 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. At the same time, the scale of turbulence as well as the scale of the magnetic field are both fractions of the virial scale, e.g. the cluster size. This is a parameter-free model (seed field is also not a parameter) and the values for the magnitude and the scale of the magnetic field are broadly consistent with observations, basically confirming the idea that the magnetic field in today's galaxy cluster is a record of its past turbulent activity.