On Three-Dimensional Spiral Galaxies

Density waves in 3D spiral galaxies are studied. In order to eliminate the forbidden region near the corotation in the grand-design galaxies, we assume that the perturbation satisfies the stable condition Q(r) > 1 over all the disk except that at the corotation. Then, a new method is put forward here to determine some basic parameters of spiral galaxies. We apply it to our Galaxy, and the results are in good agreement with the previous results.