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