Magnetic Field Evolution in Merging Clusters of Galaxies

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Abstract
We present initial results from the first three-dimensional numerical magnetohydrodynamical (MHD) simulations of magnetic field evolution in merging clusters of galaxies. Within the framework of idealized initial conditions similar to our previous work, we look at the gas dynamics and the magnetic field evolution during a major merger event in order to examine the suggestion that shocks and turbulence generated during a cluster/subcluster merger can produce magnetic field amplification and relativistic particle acceleration and, as such, may play a role in the formation and evolution of cluster-wide radio halos. The intracluster medium (ICM), as represented by the equations of ideal MHD, is evolved self-consistently within a changing gravitational potential defined largely by the collisionless dark matter component represented by an N-body particle distribution. The MHD equations are solved by the Eulerian, finite-difference code, ZEUS. The particles are evolved by a standard particle-mesh (PM) code. We find significant evolution of the magnetic field structure and strength during two distinct epochs of the merger evolution. In the first, the field becomes quite filamentary as a result of stretching and compression caused by shocks and bulk flows during infall, but only minimal amplification occurs. In the second, amplification of the field occurs more rapidly, particularly in localized regions, as the bulk flow is replaced by turbulent motions (i.e., eddies). The total magnetic field energy is seen to increase by nearly a factor of 3 over that seen in a nonmerging cluster. In localized regions (associated with high vorticity), the magnetic energy can increase by a factor of 20 or more. A power spectrum analysis of the magnetic energy shows the amplification is largely confined to scales comparable to and smaller than the cluster cores, indicating that the core dimensions define the injection scale. Although the cluster cores are numerically well-resolved, we cannot resolve the formation of eddies on scales smaller than approximately half a core radius. Consequently, the field amplification noted here likely represents a lower limit. We discuss the effects of anomalous resistivity associated with the finite numerical resolution of our simulations on the observed field amplification.
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