Galactic Rings 10.2. As was shown in the previous sections, and section 9 in particular, rings are relatively narrow features, are always associated with gas, and are usually the site of enhanced star formation. This suggests that gas is at the basis of ring formation. The high velocity dispersion of the old stellar component will prevent it from forming very sharp structures.
Gas is therefore necessary also for the persistence of a ring structure.
We have also emphasized that rings are preferentially found in barred galaxies. They have a preferential orientation with respect to the bar, either parallel or perpendicular, and their sizes are also related to that of the bar. The inner ring is always encircling the bar, forming the famous -shape. All this already tells us that the bar must be an essential element in ring formation. When a density wave of constant pattern speed is present in a galaxy, resonant phenomena with this forced pattern speed have time to develop and will be conspicuous.