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 ˜ 0.2) host enormous radio-emitting shocks with simple geometry. We investigate the role of mergers and shocks in shaping the Hα luminosity function, using custom-made narrow-band filters matching the cluster redshifts mounted on the Isaac Newton Telescope. We surveyed ˜0.28 deg2 for each cluster and found 181 line emitters in the `sausage' (volume of 3.371 × 103 Mpc3 for Hα at z = 0.1945) and 141 in the `toothbrush' (4.546 × 103 Mpc3 for Hα at z = 0.225), out of which 49 (`sausage') and 30 (`toothbrush') are expected to be Hα. We build luminosity functions for the field-of-view down to an average limiting star formation rate of 0.14 M⊙ 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 Hα 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 9σ level) in the normalization φ* 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.
