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3.4 Declining Rotation Curves

Rotation curves which drop relatively sharply beyond the optical radius and stay more or less flat thereafter, might, because of the additional identifiable feature, 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-1 in collaboration with Van Gorkom, Gunn, Knapp and Athanassoula. Several new cases of galaxies with such 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 disc 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 1.15 for a maximum disc model, but 2.30 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 disc (Thornley 1996). Therefore, it seems unlikely that the inner parts of bright disc galaxies are dark matter dominated.

Another spectacular case of a declining rotation curve has been reported for NGC 157 by Ryder et al. (1998). The strong warp and the relatively low inclination of the HI envelope make the determination of the rotation curve a bit uncertain, but there is a definite decoupling between the inner disc and the outer halo as traced by the HI.