3.2 Recent Applications of Swing Amplifier Criteria
Vogt et al. (1996) began a campaign to obtain an idea of the rotation curves of faraway galaxies, thanks to powerful equipment like the Hubble Space Telescope and the Keck Telescope. For about a dozen spirals they derive position velocity curves, which are heavily affected by aperture effects, since the slit width is non-negligible compared to the size of the galaxies. For one galaxy, 0305-00115, at z = 0.48, Fuchs et al. (1998) argue on the basis of swing amplifier criteria that a dark halo ought to be present: for a maximum disc solution the number of arms, calculated on the assumption that X = 2, is too low. However, in the rising parts of the rotation curve is less than 1.0, so that the X-value corresponding to maximum amplification is less than 2.
Mihos et al. (1997) apply the same notions of stability criteria to the Low Surface Brightness (LSB) galaxies. Since these are thought to be dark matter dominated, the amplification of m = 2 structures in those galaxies is much more difficult. Only interactions can trigger m = 2 disturbances, and even then the amplitudes are small and the deviations for axisymmetry not very large. However, the decomposition into disc and halo depends crucially on the correct determination of the rotation curve in the inner parts, and, as shown by De Blok (this meeting), the data for the LSB galaxy UGC 128 show that its disc is not necessarily dark matter dominated in the inner parts.
Figure 3. HI line width corrected for inclination according to the data given in Tully (1988) as function of arm class for the galaxies in the compilation of Elmegreen & Elmegreen (1987). The dashed line corresponds to a corrected line width of 400 km s-1.