3.2 Declining rotation curves
Some galaxies have rotation curves which decline just beyond the optical image, and stay more or less flat thereafter. Early examples are NGC 5033 and NGC 5055 in Bosma (1978, 1981a), and also NGC 5908 (Van Moorsel 1982). Two more examples, NGC 2683 and NGC 3521, were given by Casertano & Van Gorkom (1991), who speculated that declining curves are linked with disks having short scalelengths. However, Broeils (1992) finds cases of declining curves for galaxies with large disk scalelengths.
Declining rotation curves, because of the additional identifiable feature in the rotation curve, might hold out a promise to enable us to discriminate between the various mass models. Since one expects them to be found amongst galaxies with high rotational velocities, I made a small survey with the VLA of a number of galaxies with WR > 400 km/s in collaboration with Van Gorkom, Gunn, Knapp and Athanassoula. Several new cases of galaxies with declining rotation curves were found. In Bosma (1998) a preliminary account is given for the most spectacular case, NGC 4414, for which also radial velocities and velocity dispersion information was obtained.
Unfortunately, the range in disk mass-to-light ratios for that galaxy cannot be constrained very easily, in spite of the feature. However, the velocity dispersion data allow the evaluation of the Toomre Q-parameter, which is found to be about 1.1 for a maximum disk model, but 2.3 for a ``no m = 2'' model. The latter value is definitely too high to allow spiral structure from swing amplification. A weak global spiral pattern is present in the old disk (Thornley 1996). Therefore, it seems unlikely that the inner parts of bright disk galaxies are dark matter dominated.