D. Sobral, A. Stroe, W. A. Dawson, D. Wittman, M. J. Jee, H. J. A. Röttgering, R. J. van Weeren, M. Brüggen

Abstract
Cluster mergers may play a fundamental role in the formation and evolution of cluster galaxies. Stroe et al. revealed unexpected overdensities of candidate Hα emitters near the ∼1-Mpc-wide shock fronts of the massive (∼2 × 10¹⁵ M_⊙) ‘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 Hα 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 [S ii]6716/[S ii]6761 and high [S ii] 6716/Hα, implying very low electron densities (<30 lower="" than="" all="" other="" star-forming="" galaxies="" outside="" the="" cluster="" and="" or="" significant="" contribution="" from="" supernovae="" respectively="" near="" centre="" show="" evidence="" of="" outflows="" blueshifted="" na="" d="" 200="" 300="" km="" s="" sup="">−1), likely driven by supernovae. Strong outflows are also found for the cluster Hα active galactic nucleus (AGN). Hα star-forming galaxies in the merging cluster follow the z ∼ 0 mass–metallicity relation, showing systematically higher metallicity (∼0.15–0.2 dex) than Hα 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 (∼10¹⁵ M_⊙) 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 Myr.

Keywords
galaxies: clusters: general, galaxies: clusters: individual: CIZA J2242.8+5301, galaxies: evolution, intergalactic medium, cosmology: observations, large-scale structure of Universe

Monthly Notices of the Royal Astronomical Society
Volume 450, Issue 1, Page 630
2015 June

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