Transformations of galaxies. III. Encounter dynamics and tidal response
  as functions of galaxy structure
  Tidal interactions between disc galaxies depend on galaxy structure, but the
details of this relationship are incompletely understood. I have constructed a
three-parameter grid of bulge/disc/halo models broadly consistent with
$\Lambda$CDM, and simulated an extensive series of encounters using these
models. Halo mass and extent strongly influence the dynamics of orbit
evolution. In close encounters, the transfer of angular momentum mediated by
the dynamical response of massive, extended haloes can reverse the direction of
orbital motion of the central galaxies after their first passage. Tidal
response is strongly correlated with the ratio $v_\mathrm{e} / v_\mathrm{c}$ of
escape to circular velocity within the participating discs. Moreover, the same
ratio also correlates with the rate at which tidal tails are reaccreted by
their galaxies of origin; consequently, merger remnants with `twin tails', such
as NGC 7252, may prove hard to reproduce unless $(v_\mathrm{e} /
v_\mathrm{c})^2 \lesssim 5.5$. The tidal morphology of an interacting system
can provide useful constraints on progenitor structure. In particular,
encounters in which halo dynamics reverses orbital motion exhibit a distinctive
morphology which may be recognized observationally. Detailed models attempting
to reproduce observations of interacting galaxies should explore the likely
range of progenitor structures along with other encounter parameters.
