Rings and spirals in barred galaxies. II. Ring and spiral morphology
  In this series of papers, we propose a theory to explain the formation and
properties of rings and spirals in barred galaxies. The building blocks of
these structures are orbits guided by the manifolds emanating from the unstable
Lagrangian points located near the ends of the bar. In this paper we focus on a
comparison of the morphology of observed and of theoretical spirals and rings
and we also give some predictions for further comparisons. Our theory can
account for spirals as well as both inner and outer rings. The model outer
rings have the observed $R_1$, $R_1'$, $R_2$, $R_2'$ and $R_1R_2$ morphologies,
including the dimples near the direction of the bar major axis. We explain why
the vast majority of spirals in barred galaxies are two armed and trailing, and
discuss what it would take for higher multiplicity arms to form. We show that
the shapes of observed and theoretical spirals agree and we predict that
stronger non-axisymmetric forcings at and somewhat beyond corotation will drive
more open spirals. We compare the ratio of ring diameters in theory and in
observations and predict that more elliptical rings will correspond to stronger
forcings. We find that the model potential may influence strongly the numerical
values of these ratios.
