The most extreme cluster mergers can lead to massive cluster-wide travelling shock waves .
The CIZA J2242.8+5301 ( ‘ sausage ’ ) and 1RXS J0603.3+4213 ( ‘ toothbrush ’ ) clusters ( z \sim 0.2 ) host enormous radio-emitting shocks with simple geometry .
We investigate the role of mergers and shocks in shaping the H \alpha luminosity function , using custom-made narrow-band filters matching the cluster redshifts mounted on the INT .
We surveyed \sim 0.28 deg ^ { 2 } for each cluster and found 181 line emitters in the ‘ sausage ’ ( volume of 3.371 \times 10 ^ { 3 } Mpc ^ { 3 } for H \alpha at z = 0.1945 ) and 141 in the ‘ toothbrush ’ ( 4.546 \times 10 ^ { 3 } Mpc ^ { 3 } for H \alpha at z = 0.225 ) , out of which 49 ( ‘ sausage ’ ) and 30 ( ‘ toothbrush ’ ) are expected to be H \alpha .
We build luminosity functions for the field-of-view down to an average limiting star formation rate of 0.14 M _ { \sun } yr ^ { -1 } , find good agreement with field luminosity functions at z = 0.2 , but significant differences between the shapes of the luminosity functions for the two clusters .
We discover extended , tens-of-kpc-wide H \alpha haloes in galaxies neighbouring relics , which were possibly disrupted by the passage of the shock wave .
By comparing the ‘ sausage ’ cluster with blank fields and other clusters , we also uncover an order of magnitude boost ( at 9 \sigma level ) in the normalisation \phi ^ { * } of the luminosity function in the relic areas .
Our results suggest that cluster mergers may play an important role in the evolution of cluster galaxies through shock-induced star formation .