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Cluster mergers may play a fundamental role in the formation and evolution ofcluster galaxies. Stroe et al. (2014) revealed unexpected over-densities ofcandidate H$\alpha$ emitters near the ~1 Mpc-wide shock fronts of the massive(~2x10$^{15}$M$_{\odot}$) "Sausage" merging cluster, CIZA J2242.8+5301. We usedKeck/DEIMOS and WHT/AF2 to confirm 83 H$\alpha$ emitters in and around themerging cluster. We find that cluster star-forming galaxies in the hottestX-ray gas and/or in the cluster sub-cores (away from the shock fronts) showhigh [SII]6716/[SII]6761 and high [SII]6716/H$\alpha$, implying very lowelectron densities (<30x lower than all other star-forming galaxies outside thecluster) and significant contribution from supernovae, respectively. Allcluster star-forming galaxies near the cluster centre show evidence ofsignificant outflows (blueshifted Na D~200-300km/s), likely driven bysupernovae. Strong outflows are also found for the cluster H$\alpha$ AGN.H$\alpha$ star-forming galaxies in the merging cluster follow the z~0mass-metallicity relation, showing systematically higher metallicity (~0.15-0.2dex) than H$\alpha$ emitters outside the cluster (projected R>2.5 Mpc). Thissuggests that the shock front may have triggered remaining metal-rich gas whichgalaxies were able to retain into forming stars. Our observations show that themerger of impressively massive (~10$^{15}$M$_\odot$) clusters can provide theconditions for significant star-formation and AGN activity, but, as we witnessstrong feedback by star-forming galaxies and AGN (and given how massive themerging cluster is), such sources will likely quench in a few 100 Myrs.

MC$^2$: Boosted AGN and star-formation activity in CIZA J2242.8+5301, a massive post-merger cluster at z=0.19