Slowly Breaking Waves: The Longevity of Tidally Induced Spiral Structure
  We have discovered long-lived waves in two sets of numerical models of fast
(marginally bound or unbound) flyby galaxy collisions, carried out
independently with two different codes. In neither simulation set are the
spirals the result of a collision-induced bar formation. Although there is
variation in the appearance of the waves with time, they do not disappear and
reform recurrently, as seen in other cases described in the literature. We also
present an analytic theory that can account for the wave structure, not as
propagating transients, nor as a fixed pattern propagating through the disc.
While these waves propagate through the disc, they are maintained by the
coherent oscillations initiated by the impulsive disturbance. Specifically, the
analytic theory suggests that they are caustic waves in ensembles of stars
pursuing correlated epicyclic orbits after the disturbance. This theory is an
extension of that developed by Struck and collaborators for colliding ring
galaxies.
  The models suggest that this type of wave may persist for a couple of Gyr.,
and galaxy interactions occur on comparable timescales, so waves produced by
the mechanism may be well represented in observed spirals. In particular, this
mechanism can account for the tightly wound, and presumably long-lived spirals,
seen in some nearby early-type galaxies. These spirals are also likely to be
common in groups and clusters, where fast encounters between galaxies occur
relatively frequently. However, as the spirals become tightly wound, and evolve
to modest amplitudes, they may be difficult to resolve unless they are nearby.
Nonetheless, the effect may be one of several processes that result from galaxy
harassment, and via wave-enhanced star formation contribute to the
Butcher-Oemler effect.
