The information contained in 21-cm HI line maps of spiral galaxies enables us in principle to derive rotation curves over almost the entire radial extent. Most galaxies, however, as discussed in chapter 5, do not follow the ideal picture of having flat HI disks in circular motion, a picture prerequisite for a simple rotation curve analysis. Instead, several types of non circular motions can be distinguished, constituting dynamical problems by themselves. The presence of these problems makes it often difficult to derive a meaningful rotation curve, and the inference of the distribution of mass is even more problematic.
In this chapter we will treat the deviations from circular motion as small perturbations to an otherwise axisymmetric disk. In section 2 we describe the various methods used to derive rotation curves from the velocity fields; we discuss the problem of deriving a model for the mass distribution from a given rotation curve in section 3. We have calculated mass models for 24 galaxies listed in table 1 of chapter 2, which have been observed with such a beam that the ratio radius/beamsize (R/B) is larger than 5. We have also calculated a mass model for our Galaxy, based on the rotation curve given by Gordon and Burton (1976). We present the results of the mass models in section 4 and discuss their uncertainties in section 5. A detailed comparison with other physical quantities, and with morphological type, is deferred to chapter 7.