7.2. Spiral arm character, multiplicity, and disk resonances
One of the most important recent advances in morphology concerns spiral arm character. Kormendy and Norman (1979) demonstrated that spiral arm morphology depends directly on whether the disk suffers from a global instability, such as a bar, oval, or companion. In the absence of an internal or external non-axisymmetric perturbation, and in the presence of differential rotation, a galaxy will tend to have "flocculent" (or piece-meal) spiral structure, while the presence of perturbations will generally lead to well-defined global or "grand design" spiral patterns in spite of differential rotation. Elmegreen and Elmegreen (1982) proposed a system of "arm classes" to recognize this distinction and everything in between for the purpose of studying density waves in galactic disks. A summary of the physical insights derived from this scheme is given by Elmegreen and Elmegreen (1987).
An important recent use of morphology has been to identify resonance locations in galactic disks, leading to estimates of pattern speeds. In the case of pure spirals, Elmegreen, Elmegreen, and Seiden (1989) and Elmegreen and Elmegreen (1990) have used regularities (gaps, enhancements) in the arms of M 51, M 81, M 100 and NGC 1566 to trace wave resonances and evaluate the modal and stellar dynamical theories of spiral structure. In other galaxies, rings may be the most prominent tracers of specific bar resonances (Schwarz, 1979, 1981; Buta, 1986a,b). The inner and outer Lindblad, inner 4/1, and corotation resonances figure prominently in these studies because the morphological expectations of each of these resonances are fairly well-understood (see B. G. Elmegreen, 1990 and references therein). Arm multiplicity is also important for density wave studies (B. G. Elmegreen, 1990; D. M. Elmegreen, 1990), and may also shed light on resonance associations and pattern speeds.
The relationship between bars and the form of spiral structure has been an additional important topic. Kormendy (1979) demonstrated that most barred spirals have global spiral structure. Elmegreen and Elmegreen (1982) confirmed this observation and found that 79% of field barred or oval galaxies have grand design patterns. This fraction increased to more than 90% when restricted to binary galaxies.