The parameters , and have been chosen to provide a typical rotation curve, increasing linearly below and staying flat up to , beyond which it decreases smoothly. For the bulge, , and , which is equivalent to a Plummer sphere. The parameters for the visible and gas disks are respectively : , , , , , . For convenience the length unit is chosen to be the kpc and time unit the Myr. Fixing the gravitational constant to a value of 1, the mass unit corresponds to . With this set, the velocity unit is about . Fig. 1 shows the isodensity curves corresponding to the model. Well above the galaxy disk, negative density regions do exist, however our results are not spoiled by negative densities as long as orbits do not cross such regions.