The galaxy population of the complex cluster system Abell 3921
  We present a spectrophotometric analysis of the galaxy pop. in the area of
the merging cluster Abell 3921 at redshift 0.093. We investigate the impact of
the complex cluster environment on galaxy properties such as morphology or star
formation rate. We combine multi-object spectroscopy from the 2dF spectrograph
with optical imaging taken with the ESO WFI. We carry out a redshift analysis
and determine cluster velocity dispersions using biweight statistics. Applying
a Dressler-Shectman (DS-)test we seek evidence for cluster substructure.
Cluster and field galaxies are investigated with respect to [OII] and H{\alpha}
equivalent width, SFR and morphological descriptors such as concentration index
and Gini coefficient. We study these cluster galaxy properties as a function of
clustercentric distance and investigate the spatial distribution of various
galaxy types. Applying the DS-test we find a 3rd component (A3921-C) in
addition to the two main subclusters (A3921-A and A3921-B) already known. The
re-determined mass ratio between the main components A and B is approx. 2:1.
Similar to prev. studies of galaxy clusters, we find that a large fraction of
the disk galaxies close to the cluster core show no detectable star formation.
These are likely systems that are quenched due to ram pressure stripping. We
also find quenched spirals at rather large distances of 3 to 4 Mpc from the
cluster core. A3921-C might be a group of galaxies falling onto the main
cluster components. We speculate that the unexpected population of quenched
spirals at large clustercentric radii in A3921-A and A3921-B might be an effect
of the ongoing cluster merger: shocks in the ICM might give raise to enhanced
ram pressure stripping and at least in part be the cause for the quenching of
star formation. These quenched spirals might be an interm. stage in the
morphological transformation of field spirals into cluster S0s.
