Jellyfish: Ram-pressure stripping as a diagnostic tool in studies of
  cluster collisions
  Prompted by the discovery of A1758N_JFG1, a spectacular case of ram-pressure
stripping (RPS) in the galaxy cluster A1758N, we investigate the properties of
other galaxies suspected to undergo RPS in this equal-mass, post-collision
merger. Exploiting constraints derived from Hubble Space Telescope images and
Keck longslit spectroscopy, our finding of apparent debris trails and
dramatically enhanced star formation rates in an additional seven RPS
candidates support the hypothesis that RPS, and hence rapid galaxy evolution in
high-density environments, is intricately linked to cluster collisions.
Unexpectedly, we find the vast majority of RPS candidates in A1758N to be
moving toward us, and in a shared direction as projected on the plane of the
sky. We hypothesize that this directional bias is the result of two successive
events: (1) the quenching, during and after the first core passage, of star
formation in galaxies with an approximately isotropic velocity distribution
within the central region of the merger, and (2) RPS events triggered in
late-type galaxies falling into the merging system along a filament, possibly
enhanced by a shock front expanding into the outskirts of the south-eastern
subcluster. Since this explanation implies that the merger axis of A1758N must
be significantly inclined with respect to the plane of the sky, our findings
open the possibility of RPS events becoming important diagnostic tools to
constrain the geometry of cluster collisions that, due to the orientation of
the merger axis, lack the classic observational signatures of face-on mergers.
