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.
