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We propose a new theory to explain the formation of spiral arms and of alltypes of outer rings in barred galaxies. We have extended and applied thetechnique used in celestial mechanics to compute transfer orbits. Thus, ourtheory is based on the chaotic orbital motion driven by the invariant manifoldsassociated to the periodic orbits around the hyperbolic equilibrium points. Inparticular, spiral arms and outer rings are related to the presence ofheteroclinic or homoclinic orbits. Thus, R1 rings are associated to thepresence of heteroclinic orbits, while R1R2 rings are associated to thepresence of homoclinic orbits. Spiral arms and R2 rings, however, appear whenthere exist neither heteroclinic nor homoclinic orbits. We examine theparameter space of three realistic, yet simple, barred galaxy models anddiscuss the formation of the different morphologies according to the propertiesof the galaxy model. The different morphologies arise from differences in thedynamical parameters of the galaxy.

The formation of spiral arms and rings in barred galaxies