Radio signature of
structure formation shocks

The gavitationally bound structures -halos- in the Universe, like galaxies and galaxy clusters, grow by smooth accretion of matter and by merging with objects of similar size. In the aftermath of a merger a shock wave propagates outwards into the periphery of the halo. At the shock front a small fraction of the electrons is accelerated to even relativistic energies. When these electrons cool down again they emit synchrotron radiation which can be observed with radio telescopes. Some spectacular objects have been found, e.g., in Abell 3667 (Roettgering et al. 97, colors indicate the x-ray emission and contours the diffuse radio emission) and Abell 2744 (Orru et al. 07, contours represent the diffuse radio emission and colors its spectral index, a measure for age of the suprathermal electrons)

The succesion of merging events leads to a sequence of shock fronts arranged like an onion. Only a few shocks are strong enough to generate radio emission observable with present radio telescopes. The outermost shock fronts ('accretion shocks') are very strong but their radio signal is weak. They heat, e.g., the intergalactic gas in galaxy clusters ('intra-cluster medium') to its temperature above 10 billion Kelvin. Our simulations indicate that next generation telescopes will be able to detect these shocks.

Below is an animation of a merger shock front and its radio emission

Below is an animation of the evolution of a merger shock