Neutral hydrogen gas, past and future star-formation in galaxies in and around the 'Sausage' merging galaxy cluster

TL;DR

This study investigates how a massive merging galaxy cluster affects the neutral hydrogen gas reservoirs in its galaxies, revealing that these galaxies retain substantial gas and sustain star formation despite the merger and shock activity.

Contribution

Deep HI observations of the 'Sausage' cluster show that member galaxies retain significant gas, challenging previous assumptions about gas depletion during cluster mergers.

Findings

Cluster galaxies have similar HI content to field galaxies when accounting for mass.

Star formation correlates with supernova-related radio emission, indicating sustained star formation post-shock.

Gas reservoirs are expected to be consumed within 0.75-1.0 Gyr due to star formation and outflows.

Abstract

CIZA J2242.8+5301 ($z = 0.188$, nicknamed 'Sausage') is an extremely massive ($M_{200}\sim 2.0 \times 10^{15}M_\odot$ ), merging cluster with shock waves towards its outskirts, which was found to host numerous emission-line galaxies. We performed extremely deep Westerbork Synthesis Radio Telescope HI observations of the 'Sausage' cluster to investigate the effect of the merger and the shocks on the gas reservoirs fuelling present and future star formation (SF) in cluster members. By using spectral stacking, we find that the emission-line galaxies in the 'Sausage' cluster have, on average, as much HI gas as field galaxies (when accounting for the fact cluster galaxies are more massive than the field galaxies), contrary to previous studies. Since the cluster galaxies are more massive than the field spirals, they may have been able to retain their gas during the cluster merger. The large HI reservoirs are expected to be consumed within $\sim0.75-1.0$ Gyr by the vigorous SF and AGN activity and/or driven out by the out-flows we observe. We find that the star-formation rate in a large fraction of H$\alpha$ emission-line cluster galaxies correlates well with the radio broad band emission, tracing supernova remnant emission. This suggests that the cluster galaxies, all located in post-shock regions, may have been undergoing sustained SFR for at least 100 Myr. This fully supports the interpretation proposed by Stroe et al. (2015) and Sobral et al. (2015) that gas-rich cluster galaxies have been triggered to form stars by the passage of the shock.