2.1 ``Wiggles''

One argument for the maximum disc hypothesis is the presence of ``wiggles'' in the rotation curves, in particular those derived from long slit spectroscopy. This was already remarked upon by Kent (1986) and Van Albada & Sancisi (1986). These wiggles can, remarkably, be fitted using the maximim disc approach, even though it is quite likely that the ``wiggles'' in the position - velocity diagram resulting directly from the spectrocopy are due to the crossing of the spiral arms, as in the case of NGC 2998 (cf. Rubin et al. 1978). Freeman (1992) likewise shows a few cases from the work of Buchhorn (1992) using the Mathewson et al. (1992) data.

However, when the motions giving rise to the ``wiggles'' are non-circular motions due to the spiral arms, it is technically incorrect to speak of rotation curves here. Visser (1980), in his study of M81, shows that the average rotation curve of a two-dimensional velocity field strongly perturbed by motions due to the spiral arms is wiggly, while the ``true'' rotation curve he needs to fit such a velocity field with a model based on density wave streaming motions is smooth and without ``wiggles''. Thus the ``wiggle'' argument is not usable in the context of rotation curves. In any case, Van der Kruit (1995) shows that also for non-maximum discs the wiggles can be fitted.

The only rational way to reformulate the ``wiggle'' argument, as proposed by Binney (priv. comm.), is to consider that, since they are there and due to spiral arm streaming motions, there should be a non-negligible part of the mass in the disc. A similar argument is made by Sellwood (this volume) concerning the capability of fitting gas flow models of barred spirals to high resolution two dimensional velocity fields.