GASP XXIX -- Unwinding the arms of spiral galaxies via ram-pressure
  stripping
  We present the first study of the effect of ram-pressure "unwinding" the
spiral arms of cluster galaxies. We study 11 ram-pressure stripped galaxies
from GASP (GAs Stripping Phenomena in galaxies) in which, in addition to more
commonly observed "jellyfish" features, dislodged material also appears to
retain the original structure of the spiral arms. Gravitational influence from
neighbours is ruled out and we compare the sample with a control group of
undisturbed spiral galaxies and simulated stripped galaxies. We first confirm
the unwinding nature, finding the spiral arm pitch angle increases radially in
10 stripped galaxies and also simulated face-on and edge-on stripped galaxies.
We find only younger stars in the unwound component, while older stars in the
disc remain undisturbed. We compare the morphology and kinematics with
simulated ram-pressure stripping galaxies, taking into account the estimated
inclination with respect to the intracluster medium and find that in edge-on
stripping, unwinding can occur due to differential ram-pressure caused by the
disc rotation, causing stripped material to slow and "pile-up". In face-on
cases, gas removed from the outer edges falls to higher orbits, appearing to
"unwind". The pattern is fairly short-lived (<0.5Gyr) in the stripping process,
occurring during first infall and eventually washed out by the ICM wind into
the tail of the jellyfish galaxy. By comparing simulations with the observed
sample, we find a combination of face-on and edge-on "unwinding" effects are
likely to be occurring in our galaxies as they experience stripping with
different inclinations with respect to the ICM.
