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The 2012 Phd Award of the Astronomische Gesellschaft goes to Julius Donnert for his work on Giant Radio Halos

 PhD award of the AG 2012
Radio Halos are Mpc sized diffuse objects observed exclusively in merging clusters of galaxies. This non-thermal emission is due to relativistic electrons interacting with the intra-cluster magnetic field. The injection of these particles in shocks and galactic outflows is believed to be localised. At the same time the life-time of synchrotron bright electrons in the ICM is short. Therefore the sheer size of radio halos poses the puzzle to how nature maintains a cluster-wide CR electron population. Hadronic models solve this problem with in-situ injection of CR electrons through hadronic interactions of long lived CR protons with the ICM. However, over the past years it has become clear that these models are disfavoured by observations. A more complete approach, reacceleration models, are predicted to elegantly solve many problems encountered with other models, at the cost of increased complexity. 

We present a study based on constrained cosmological MHD simulations and an analytic description for steady-state CR protons and electrons in the ICM. We find that pure hadronic models are not able to explain the full complexity of observed radio halo characteristics. Specifically the transient nature of the emission (on-off) and the curved non-thermal spectra found in the COMA cluster. We predict upper limits in the gamma-ray regime for the most extreme of these models. Today these models are ruled out by recent non-detections of the COMA cluster by the FERMI satellite. In a second step we use a novel implementation of a Fokker-Planck solver to follow the dynamics of CR electrons in postprocessing to a direct simulation of a cluster merger. As an explorative approach we use hadronic injection of CR electrons to seed the population. We find that this mechanism does not only produce transient emission, but also gives the complex non-thermal radio spectra observed in some clusters and CR proton densities compatible with recent gamma-ray observations. 
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Highlights

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Julius Donnert receives PhD award of AG Phd Award 2012 of the Astronomische Gesellschaft
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