Galaxy evolution in merging clusters: The passive core of the "Train
  Wreck" cluster of galaxies, A520
  The mergers of galaxy clusters are the most energetic events in the universe
after the Big Bang. With the increased availability of multi-object
spectroscopy and X-ray data an ever increasing fraction of local clusters are
recognised as exhibiting signs of recent or past merging events on various
scales. Our goal is to probe how these mergers affect the evolution and content
of their member galaxies. We specifically aim to answer the following
questions: Is the quenching of star formation in merging clusters enhanced when
compared with relaxed clusters? Is the quenching preceded by a (short lived)
burst of star formation? We obtained optical spectroscopy of >400 galaxies in
the field of the merging cluster Abell 520. We combine these observations with
archival data to get a comprehensive picture of the state of star formation in
the members of this merging cluster. Finally, we compare these observations
with a control sample of 10 non-merging clusters at the same redshift from The
Arizona Cluster Redshift Survey (ACReS). We split the member galaxies in
passive, star forming or recently quenched depending on their spectra. The core
of the merger shows a decreased fraction of star forming galaxies compared to
clusters in the non-merging sample. This region, dominated by passive galaxies,
is extended along the axis of the merger. We find evidence of rapid quenching
of the galaxies during the core passage with no signs of a star burst on the
time scales of the merger (~0.4Gyr). Additionally, we report the tentative
discovery of an infalling group along the main filament feeding the merger,
currently at ~2.5 Mpc from the merger centre. This group contains a high
fraction of star forming galaxies as well as ~2/3 of all the recently quenched
galaxies in our survey.
