3.3 Flocculent and Grand Design Galaxies

Elmegreen & Elmegreen (1990) made a statistic based on an extended gradient in the rotation curve, and compared this with their arm classes (Elmegreen & Elmegreen 1982). They concluded that there is a correlation between the two, in the sense that declining rotation curves are found in grand design spirals. From this they concluded that grand design spirals have smaller dark halos than flocculent galaxies.

Their interpretation, however, rests on their mixing large and small galaxies into only three bins, flocculent galaxies, intermediate galaxies, and grand design galaxies. A more detailed analysis can be done if one considers the width of the HI profile, corrected for inclination, with arm class. This has been done in Figure 3 for all the galaxies for which arm classes are available in Elmegreen & Elmegreen (1987) and HI profile data in Tully's (1988) Nearby Galaxy Catalog. It can be seen that the galaxies in arm class 3 have high rotational velocitites, which corresponds with their regular structure. Combining arm classes 1-4 into one bin called ``flocculent'' thus lumps together dwarf galaxies with rising curves and giant galaxies with slowly declining curves. The ``grand design'' galaxies (arm classes 9 & 12) are predominantly giant galaxies, and the intermediate classes (arm classes 5-8) also. In view of these biases, Elmegreen & Elmegreen's (1990) conclusions cannot be sustained.