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K. Roettiger, J. O. Burns (University of Missouri-Columbia)
We present preliminary results from our numerical study of radio halo formation and evolution in the cluster merger environment. Radio halos are large (> 0.5 Mpc), amorphous, and diffuse radio sources which appear to be intrinsic to the cluster itself rather than any particular galaxy within the cluster. The difficulty in explaining radio halos is a result of their large size which implies either unusually high diffusion rates necessary to transport the relativistic particles large distances during their relatively short radiative lifetimes or in situ particle acceleration over comparably large regions. Recently, it has been noted that radio halos are typically associated with more luminous clusters which appear to have undergone recent evolution, specifically mergers with other clusters. Cluster mergers generate significant hydrodynamics (shocks, bulk flows, and turbulence) capable of amplifying magnetic fields and accelerating relativistic particles, both of which are necessary to produce the observed synchrotron emission. We find that mergers may provide two distinct phases of halo evolution having significant implications for the observed radio morphology. The first phase is dominated by shock acceleration and high velocity transport of relativistic particles by bulk flows and may result in halo/relic emission found in young mergers (e.g. A3667 and A2256). The second phase is dominated by turbulent particle acceleration and may explain more diffuse sources such as that found in Coma.