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

TL;DR

This study confirms enhanced star formation and AGN activity in a massive merging galaxy cluster, revealing how cluster mergers influence galaxy evolution through triggered star formation and feedback processes.

Contribution

It provides the first spectroscopic confirmation of Hα emitters in CIZA J2242.8+5301 and analyzes their properties, linking merger dynamics to galaxy activity.

Findings

Cluster star-forming galaxies show low electron densities and supernova-driven outflows.

Galaxies near the cluster center have higher metallicity, indicating shock-triggered metal retention.

Strong feedback from star formation and AGN may quench activity within a few hundred million years.

Abstract

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.