10.2. Fundamental Clues from Observations
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. Even after young stars have formed out of the gas, they will acquire velocity dispersion in a time-scale of 109 yrs, and will no longer follow a thin ring morphology. 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. Bars can be considered as long-lived density waves, and long-lasting modes. 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. The idea of rings formed at resonances with the bar is therefore entirely natural.
A detailed color decomposition of an inner ring (Buta 1991; see section 9.4) has shown that the ring is the superposition of two gaussians: a narrow blue one, containing the more recent stars, and a wider red one. The latter has about twice the width of the former. The mean central positions of the two gaussians coincide (see Figure 40), which means that the position of the inner ring has not significantly evolved from the time of its formation. The underlying pattern speed must therefore be nearly constant over the ring formation time.