In this chapter we will describe a number of general properties of the 21-cm line maps obtained for the galaxies listed in table 1 of chapter 2. An attempt at such a description is motivated by the desire to compare various galaxies of different morphological types. If this comparison were to be restricted to the radial distribution of mass surface density of neutral hydrogen gas and to the rotation curve only, a large amount of information contained in the two-dimensional maps would be ignored. Moreover, the radial distributions themselves are sometimes heavily influenced by the assumption that the two-dimensional maps have axial symmetry.
The radial velocity fields of the spirals listed in table 2.1 do indeed all have the characteristic pattern expected for an inclined disk in differential rotation, but most of them show in addition specific patterns of non-circular motions, and small scale irregularities. Often there is a strong resemblance in the type of non-circular motions among different galaxies, although their amplitude varies. These types of non-circular motions constitute not only dynamical problems by themselves, but also restrain the usefulness of the derived rotation curves.
In section 2 of this chapter we briefly describe the main types of non-circular motions. In the next five sections we discuss each type in turn; we illustrate which galaxies show these non-circular motions and we indicate which mechanisms appear to be relevant for the cause of these motions. In each section comments on the HI column density distributions of the galaxies are made when appropriate.