The rise and fall of star-formation in $\bf z\sim0.2$ merging galaxy
  clusters
  CIZA J2242.8+5301 (`Sausage') and 1RXS J0603.3+4213 (`Toothbrush') are two
low-redshift ($z\sim0.2$), massive ($\sim2\times10^{15}M_\odot$), post-core
passage merging clusters, which host shock waves traced by diffuse radio
emission. To study their star-formation properties, we uniformly survey the
`Sausage' and `Toothbrush' clusters in broad and narrow band filters and select
a sample of $201$ and $463$ line emitters, down to a rest-frame equivalent
width ($13${\AA}). We robustly separate between H$\alpha$ and higher redshift
emitters using a combination of optical multi-band (B, g, V, r, i, z) and
spectroscopic data. We build H$\alpha$ luminosity functions for the entire
cluster region, near the shock fronts, and away from the shock fronts and find
striking differences between the two clusters. In the dynamically younger, $1$
Gyr old `Sausage' cluster we find numerous ($59$) H$\alpha$ emitters above a
star-formation rate (SFR) of $0.17$ M$_{\sun}$ yr$^{-1}$ surprisingly located
in close proximity to the shock fronts, embedded in very hot intra-cluster
medium plasma. The SFR density for the cluster population is at least at the
level of typical galaxies at $z\sim2$. Down to the same star-formation rate,
the possibly dynamically more evolved `Toothbrush' cluster has only $9$
H$\alpha$ galaxies. The cluster H$\alpha$ galaxies fall on the SFR-stellar mass
relation $z\sim0.2$ for the field. However, the `Sausage' cluster has an
H$\alpha$ emitter density $>20$ times that of blank fields. If the shock passes
through gas-rich cluster galaxies, the compressed gas could collapse into dense
clouds and excite star-formation for a few $100$ Myr. This process ultimately
leads to a rapid consumption of the molecular gas, accelerating the
transformation of gas-rich field spirals into cluster S0s or ellipticals.
