18. SINGLE RINGED, MULTI-ARMED SPIRAL GALAXIES
We have shown that orbit resonance theory provides a good explanation of the morphology and other characteristics of rings seen in many early-type barred galaxies. Often, such galaxies have more than one type of ring in co-existence as we discussed in section 14.1, and the outer spiral pattern is defined by two main arms. Yet, there exist many examples of strong rings where the spiral structure beyond the ring is multi-armed. Examples illustrated here include NGC 6902 (Figure 4), IC 5240 (Figure 8), NGC 2835 and 3124 (Figure 11), and NGC 7217 (Figure 5 and Figure 37). Other strong cases include NGC 1640, 2307, 3450, and 3660 illustrated in Buta (1995). Except for NGC 1640, the outer spiral patterns in these galaxies do not resemble one of the ``OLR subclasses'', and one may well question whether the inner rings in these cases are connected to the inner 4 / 1 resonance. In most cases, there is either an obvious or a very weak bar inside the inner ring.
Multi-armed and flocculent galaxies have been reviewed by Elmegreen (1991) and simulated by Elmegreen & Thomasson (1993). The study by Combes & Elmegreen (1993) also provided some insight into these kinds of galaxies. Elmegreen (1991) points out that multi-armed galaxies have stellar spiral waves as seen in near-IR images, while flocculent spirals may not. He suggests that the former may either not have simple modes or may have several modes, while flocculent galaxies have such a high Q parameter that they cannot support stellar spirals while the gas can. Elmegreen & Thomasson (1993) used computer simulations to examine the effects of a Q barrier (a radius where Q increases rapidly) on the spiral structure. With a Q-barrier, well-developed two-armed spirals resulted, but without one, multi-armed and flocculent spirals were obtained. The difference between the latter two types of spirals was connected to the Q value of the stars. In an early-type galaxy simulation with gas, Combes & Elmegreen (1993) obtained nuclear and inner gas rings at the ILR and inner 4 / 1 UHR, but a multiple-armed, almost flocculent spiral pattern outside the ring. In this case, they suggested that the bar was not strong enough to drive a coherent spiral outside CR.
It is likely that the inner rings seen in single-ringed, multi-armed or flocculent galaxies may be a mixture of inner 4 / 1 UHR and ILR ring types. The inner ring of NGC 2307 (Buta 1995) shows minor axis breaks, much like a simulated 4 / 1 resonance ring. On the other hand, the turnover radius location of the inner ring of NGC 7531, which is very much like NGC 6902 and others in this category, suggests that the ring in that case is linked to an ILR. Thus, rotation curves may help to eludicate the nature of rings in such galaxies.