THICK SPIRALS - DYNAMICS AND ORBITAL BEHAVIOR

The effect of thickness of spiral arms in the dynamics of normal spira l galaxies is examined. We find stable 3D periodic orbits with ellipti cal projections on the equatorial plane, which can support a spiral pa ttern with variable thickness. It is shown, that in general these 3D o rbits can be found even in models with strong forcing in the z-directi on. Differences depending on disk scale lengths and heights as well as on the disk/halo decomposition are outlined. The edge-on shapes of or bits found in the thick spirals are associated with the vertical reson ances existing in the potential. The structure of the stable families of periodic orbits suggests that edge-on peanut-shaped profiles are as sociated to the vertical resonances of the axisymmetric potential and therefore can appear in all types of disk galaxies i.e. not only in sy stems with a bar perturbation.