A large H$\alpha$ survey of star formation in relaxed and merging galaxy cluster environments at $z\sim0.15-0.3$

TL;DR

This study presents the largest Hα survey of star formation in galaxy clusters, revealing differences in star formation activity and galaxy environment between relaxed and merging clusters at redshift 0.15-0.3.

Contribution

It provides the first large-scale comparison of star formation in relaxed versus merging galaxy clusters using Hα emission, highlighting environmental impacts on galaxy evolution.

Findings

Merging clusters have higher Hα luminosity function density than relaxed clusters.

Hα emitter density decreases from cluster core to outskirts in both types.

Clusters with radio relics show higher star formation activity than those with radio haloes.

Abstract

We present the first results from the largest H$\alpha$ survey of star formation and AGN activity in galaxy clusters. Using 9 different narrow band filters, we select $>3000$ H$\alpha$ emitters within $19$ clusters and their larger scale environment over a total volume of $1.3\times10^5$ Mpc$^3$. The sample includes both relaxed and merging clusters, covering the $0.15-0.31$ redshift range and spanning from $5\times10^{14}$ $M_{\odot}$ to $30\times10^{14}$ $M_{\odot}$. We find that the H$\alpha$ luminosity function (LF) for merging clusters has a higher characteristic density $\phi^*$ compared to relaxed clusters. $\phi^*$ drops from cluster core to cluster outskirts for both merging and relaxed clusters, with the merging cluster values $\sim0.3$ dex higher at each projected radius. The characteristic luminosity $L^*$ drops over the $0.5-2.0$ Mpc distance from the cluster centre for merging clusters and increases for relaxed objects. Among disturbed objects, clusters hosting large-scale shock waves (traced by radio relics) are overdense in H$\alpha$ emitters compared to those with turbulence in their intra-cluster medium (traced by radio haloes). We speculate that the increase in star formation activity in disturbed, young, massive galaxy clusters can be triggered by interactions between gas-rich galaxies, shocks and/or the intra-cluster medium, as well as accretion of filaments and galaxy groups. Our results indicate that disturbed clusters represent vastly different environments for galaxy evolution compared to relaxed clusters or average field environments.