2017MNRAS.465.2916S

We present the first results from the largest Hα survey of star formation and active galactic nucleus activity in galaxy clusters. Using nine different narrow-band filters, we select > 3000 Hα emitters within 19 clusters and their larger scale environment over a total volume of 1.3 x 10⁵ Mpc³. The sample includes both relaxed and merging clusters, covering the 0.15-0.31 redshift range and spanning from 5 x 10¹⁴ to 30 x 10¹⁴ M_☉. We find that the Hα luminosity function for merging clusters has a higher characteristic density ph* compared to relaxed clusters. ph* drops from cluster core to cluster outskirts for both merging and relaxed clusters, with the merging cluster values ∼0.3 dex higher at each projected radius. The characteristic luminosity L* drops over the 0.5-2.0 Mpc distance from the cluster centre for merging clusters and increases for relaxed objects. Among disturbed objects, clusters hosting large-scale shock waves (traced by radio relics) are overdense in Hα emitters compared to those with turbulence in their intracluster medium (traced by radio haloes). We speculate that the increase in star formation activity in disturbed, young, massive galaxy clusters can be triggered by interactions between gas-rich galaxies, shocks and/or the intracluster medium, as well as accretion of filaments and galaxy groups. Our results indicate that disturbed clusters represent vastly different environments for galaxy evolution compared to relaxed clusters or average field environments.