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Description
Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. (2015MNRAS.450..646S) revealed unexpected overdensities of candidate H{alpha} emitters near the ~1-Mpc-wide shock fronts of the massive (~2x10^15^M_{sun}_) 'Sausage' merging cluster, CIZA J2242.8+5301. We used the Keck/Deep Imaging Multi-Object Spectrograph and the William Herschel Telescope/AutoFib2+WYFFOS to confirm 83 H{alpha} emitters in and around the merging cluster. We find that cluster star-forming galaxies in the hottest X-ray gas and/or in the cluster subcores (away from the shock fronts) show high [SII]6716/[SII]6761 and high [SII] 716/H{alpha}, implying very low electron densities (<30xlower than all other star-forming galaxies outside the cluster) and/or significant contribution from supernovae, respectively. All cluster star-forming galaxies near the cluster centre show evidence of significant outflows (blueshifted Na D ~ 200-300km/s), likely driven by supernovae. Strong outflows are also found for the cluster H{alpha} active galactic nucleus (AGN). H{alpha} star-forming galaxies in the merging cluster follow the z~0 mass-metallicity relation, showing systematically higher metallicity (~0.15-0.2dex) than H{alpha} emitters outside the cluster (projected R>2.5Mpc). This suggests that the shock front may have triggered remaining metal-rich gas which galaxies were able to retain into forming stars. Our observations show that the merger of impressively massive (~10^15^M_{sun}_) clusters can provide the conditions for significant star formation and AGN activity, but, as we witness strong feedback by star-forming galaxies and AGN (and given how massive the merging cluster is), such sources will likely quench in a few 100Myr.
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