The role of cluster mergers and travelling shocks in shaping the Halpha luminosity function at z~0.2: 'sausage' and 'toothbrush' clusters

Abstract

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 {\tilde} 0.2) host enormous radio-emitting shocks with simple geometry. We investigate the role of mergers and shocks in shaping the H{$α$} luminosity function, using custom-made narrow-band filters matching the cluster redshifts mounted on the Isaac Newton Telescope. We surveyed {\tilde}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{$α$} at z = 0.1945) and 141 in the `toothbrush' (4.546 {\times} 10$^{3}$ Mpc$^{3}$ for H{$α$} at z = 0.225), out of which 49 (`sausage') and 30 (`toothbrush') are expected to be H{$α$}. 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 H{$α$} 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{$σ$} level) in the normalization {ϕs}* 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.