Turbulence Dynamo in the Stratified Medium of Galaxy Clusters

Abstract

The existence of microgauss magnetic fields in galaxy clusters has been established through observations of synchrotron radiation and Faraday rotation. They are conjectured to be generated via small-scale dynamo by turbulent flow motions in the intracluster medium (ICM). The microgauss magnetic fields of giant radio relics, show structures of synchrotron polarization vectors, organized over scales of megaparsecs, challenging the turbulence origin of cluster magnetic fields. Unlike turbulence in the interstellar medium, turbulence in the ICM is subsonic. And it is driven sporadically in highly stratified backgrounds, when major mergers occur during the hierarchical formation of clusters. To investigate quantitatively the characteristics of a turbulence dynamo in such an ICM environment, we performed a set of turbulence simulations using a high-order-accurate, magnetohydrodynamic (MHD) code. We find that turbulence dynamo could generate the cluster magnetic fields up to the observed level from the primordial seed fields of 10(-15) G or so within the age of the universe, if the MHD description of the ICM could be extended down to kiloparsec scales. However, highly organized structures of polarization vectors, such as those observed in the Sausage relic, are difficult to reproduce through the shock compression of turbulence-generated magnetic fields. This implies that the modeling of giant radio relics may require pre-existing magnetic fields organized over megaparsec scales.