Ubiquitous signs of interactions in early-type galaxies with prolate
  rotation
  A small fraction of early-type galaxies (ETGs) show prolate rotation, i.e.
they rotate around their long photometric axis. In simulations, certain
configurations of galaxy mergers are known to produce this type of rotation. We
investigate the association of prolate rotation and signs of galaxy
interactions among the observed galaxies. We collected a sample of 19 nearby
ETGs with distinct prolate rotation from the literature and inspected their
ground-based deep optical images for interaction signs - 18 in archival images
and one in a new image obtained with the Milankovi\'c telescope. Tidal tails,
shells, asymmetric/disturbed stellar halos, or on-going interactions are
present in all the 19 prolate rotators. Comparing this with the frequency of
tidal disturbance among the general sample of ETGs of a roughly similar mass
range and surface-brightness limit, we estimate that the chance probability of
such an observation is only 0.00087. We also found a significant overabundance
of prolate rotators that are hosting multiple stellar shells. The visible tidal
features imply a relatively recent galaxy interaction. That agrees with the
Illustris large-scale cosmological hydrodynamical simulation, where prolate
rotators are predominantly formed in major mergers during the last 6 Gyr. In
the appendix, we present the properties of an additional galaxy, NGC 7052, a
prolate rotator for which no deep images are available, but for which an HST
image revealed the presence of a prominent shell, which had not been reported
before.
