Rings and spirals in barred galaxies. III. Further comparisons and links
  to observations
  In a 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, the last of
the series, we present more comparisons of our theoretical results to
observations and also give new predictions for further comparisons. Our theory
provides the right building blocks for the rectangular-like bar outline and for
ansae. We consider how our results can be used to give estimates for the
pattern speed values, as well as their effect on abundance gradients in barred
galaxies. We present the kinematics along the manifold loci, to allow
comparisons with the observed kinematics along the ring and spiral loci. We
consider gaseous arms and their relations to stellar ones. We discuss several
theoretical aspects and stress that the orbits that constitute the building
blocks of the spirals and rings are chaotic. They are, nevertheless, spatially
well confined by the manifolds and are thus able to outline the relevant
structures. Such chaos can be termed `confined chaos' and can play a very
important role in understanding the formation and evolution of galaxy
structures and in galactic dynamics in general. This work, in agreement with
several others, argues convincingly that galactic dynamic studies should not be
limited to the study of regular motions and orbits.
