GASP IX. Jellyfish galaxies in phase-space: an orbital study of intense
  ram-pressure stripping in clusters
  It is well known that galaxies falling into clusters can experience gas
stripping due to ram-pressure by the intra-cluster medium (ICM). The most
spectacular examples are galaxies with extended tails of optically-bright
stripped material known as "jellyfish". We use the first large homogeneous
compilation of jellyfish galaxies in clusters from the WINGS and OmegaWINGS
surveys, and follow-up MUSE observations from the GASP MUSE programme to
investigate the orbital histories of jellyfish galaxies in clusters and
reconstruct their stripping history through position vs. velocity phase- space
diagrams. We construct analytic models to define the regions in phase-space
where ram-pressure stripping is at play. We then study the distribution of
cluster galaxies in phase-space and find that jellyfish galaxies have on
average higher peculiar velocities (and higher cluster velocity dispersion)
than the overall population of cluster galaxies at all clustercentric radii,
which is indicative of recent infall into the cluster and radial orbits. In
particular, the jellyfish galaxies with the longest gas tails reside very near
the cluster cores (in projection) and are moving at very high speeds, which
coincides with the conditions of the most intense ram-pressure. We conclude
that many of the jellyfish galaxies seen in clusters likely formed via fast
(~1- 2 Gyr), incremental, outside-in ram-pressure stripping during first infall
into the cluster in highly radial orbits.
