Spectral and morphological signatures of cluster merger shocks: CIZA   J2242.8+5301
  The CIZA J2242.8+5301 cluster hosts an extraordinary, narrow, Mpc-wide, diffuse patch of radio emission tracing travelling shock waves, called a relic. We perform a multi-wavelength of observations of the cluster. We discover radio spectral steepening and increasing spectral curvature in the shock downstream area, indicative of emission produced by spectrally-aged electrons, following a JP ageing model. The curved integrated spectrum towards $16$ GHz challenges the favoured relic formation model. Further, we find a boosting the number density of H$\alpha$ emitters by a factor of $10$, possibly caused by interactions of the galaxies with the shock wave. 
Andra Stroe, Reinout J. van Weeren, David Sobral, Clare Rumsey, Huib   T. Intema, Huub J. A. R\"ottgering, Jeremy J. Harwood...
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