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From a survey of all published rotation curves, it is found that in general a galaxy shows a global spiral pattern either by maintaining nearly solid-body rotation throughout the region of the spiral pattern, or by having a density-wave driving mechanism in the form of a bar or a companion. It is reported that of the 54 galaxies studied, 21 have neither bars nor companions. Of these, nine rotate differentially but lack global patterns, while 10 have global patterns ending where the rotation curve becomes flat. In the latter galaxies the absence of an inner Lindblad resonance, which results from the nearly rigid rotation, greatly simplifies the problem of creating waves by means of disk instabilities. Finally, criteria for identifying oval distortions are discussed and examples are illustrated.
Kormendy et al. (Mon,) studied this question.