MC$^2$: Boosted AGN and star-formation activity in CIZA J2242.8+5301, a
  massive post-merger cluster at z=0.19
  Cluster mergers may play a fundamental role in the formation and evolution of
cluster galaxies. Stroe et al. (2014) revealed unexpected over-densities of
candidate 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 used
Keck/DEIMOS and WHT/AF2 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 sub-cores (away from the shock fronts) show
high [SII]6716/[SII]6761 and high [SII]6716/H$\alpha$, implying very low
electron densities (<30x lower than all other star-forming galaxies outside the
cluster) and 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$\alpha$ AGN.
H$\alpha$ star-forming galaxies in the merging cluster follow the z~0
mass-metallicity relation, showing systematically higher metallicity (~0.15-0.2
dex) than H$\alpha$ emitters outside the cluster (projected R>2.5 Mpc). 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$_\odot$) 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 100 Myrs.
