Galaxies show a large variety in form and content, and several schemes have been developed to classify their morphological properties, with the aim to relate those to physically significant ones (see Sandage, 1975). The two basic classes of galaxies are the ellipticals and the disk galaxies (Sandage, Freeman and Stokes, 1970). Disk galaxies generally possess a nuclear bulge, which is similar to ellipticals in many respects, and a disk in which usually spiral structure can be seen. Disk galaxies without spiral structure are classified as lenticulars (S0's), the remaining ones are called spirals. Unlike the ellipticals, which form a homogeneous class of galaxies, the spirals can be subdivided according to several, often complementary, criteria. This has led to the distinction of Hubble types (Sa, Sb, Sc, Sd, Sm), ordinary and barred spirals (SA and SB), and varieties (ring and spiral variety). Many spirals have been classified according to this scheme, or slightly more elaborate versions of it (De Vaucouleurs et al., 1976). Other classification criteria have been: the morphology of the arms, which is related to the luminosity (Van den Bergh, 1960), and the degree of concentration of light towards the centre, which is related to the integrated spectral type (Morgan, 1958, 1959). Irregular galaxies of the magellanic type (Im) form the natural endpoint of the so-called Hubble sequence E, So, Sa, Sb, Sc, Sd, Sm, Im.
These classifications are widely used nowadays, but their physical significance is still poorly understood. In this thesis we try to make a contribution to a better understanding of the physical processes underlying the morphology of a spiral galaxy. We have obtained new information, especially on the kinematical properties, for a number of spirals of various morphological types which enabled us to approach this problem in a new context.
Various attempts have already been made to find a physical basis for the classification schemes of spiral galaxies. One approach has been to study the integral properties, like mass, luminosity, hydrogen mass, mean colour, integrated spectral type, etc. Various loose correlations of these properties with morphological type have been found (see De Vaucouleurs, 1977 for a review). For example the ratio of neutral hydrogen mass to total mass increases along the sequence Sa-Im. In general the later type galaxies (i.e. galaxies at the end of the Hubble sequence) have relatively more population I material (gas, HII regions, OB stars) than the earlier type galaxies. The physical reason for this is unknown at present. Another correlation is the one between luminosity and the maximum rotational velocity of a galaxy (Tully and Fisher, 1977), which finds an application in distance determinations. Again the physical reason for this correlation is not entirely clear. The existence of such correlations, however, suggests that part, if not all, of the properties of spiral galaxies can be related to certain basic quantities which determine the morphology, content and dynamics of spiral galaxies.
The loose correlations found between some of the integral properties of spiral galaxies have occasionally been further interpreted by making inferences about the radial variations of several dynamical and physical parameters within each galaxy (see e.g. Shostak, 1977 for a recent attempt). In those cases a simple model is adopted for a spiral, consisting of a spheroidal component, representing the bulge, and a disk in differential circular motion. The spiral structure is treated as a perturbation superimposed on a smoothly varying radial distribution of matter, and differences among these radial distributions in various galaxies are correlated with morphological type. Information on the radial variations of several physical quantities is therefore of great interest for the study of the problem of the physical significance of the classification schemes. In principle one would like to know the radial distributions of the various stellar and gaseous constituents of galaxies of various morphological types, and also their dynamics. This represents a large and complex observational programme, however, of which the first results are only now becoming available.
At present information is available on the radial distribution of HII regions (Hodge, 1974), rotational velocities derived from optical spectroscopy and 21 cm line data (Burbidge and Burbidge, 1975; Roberts, 1975; Huchtmeier, 1975), ratios of metallic line strength in HII regions (Searle, 1971; Webster, 1977), radio continuum emission (Van der Kruit and Allen, 1976), radial luminosity profiles (Freeman, 1970), and a number of other properties. In most cases a sample of about a dozen galaxies has been studied to derive the information on one particular property, but there is little overlap among these samples. The information obtained with optical techniques is usually limited to the inner parts of a galaxy (out to 5-10 kpc), and the interpretation of most of the data obtained at radio wavelengths is severely hampered by the low spatial resolution of the telescopes.
Twenty one centimeter line observations contain information on the distribution of neutral atomic hydrogen (HI) and on the kinematics of this gas. Since HI emission can be detected over almost the entire radial extent of a spiral galaxy such data are a powerful source of information on the kinematics and dynamics of spirals. Until the early 1970's only for a few galaxies 21 cm line data were available at a resolution such that the ratio (Holmberg) radius/ beamsize is larger than 5. And, because the HI emission is stronger in late type galaxies, the distribution of these "well-studied" galaxies among the morphological types is biased in favour of these late types. Nevertheless, even from this small amount of data inferences have been made concerning the mass distributions in galaxies of different types (Roberts and Rots, 1973) and the differences between galaxies of the same type (Rogstad and Shostak, 1972).
A major step forwards in the study of those radial distributions in spirals that can be derived from data at radio wavelengths is the use of aperture synthesis instruments. In particular the Westerbork Synthesis Radio Telescope (WSRT) provides data with sufficient spatial resolution and sensitivity to obtain detailed 21 cm HI line maps of spirals of various morphological types. In this thesis we will discuss a sample of galaxies of various morphological types for which 21 cm line data are available at a spatial resolution such that the ratio radius/beamsize is larger than about 5. For a number of galaxies we have used information from the literature, for other galaxies we have used information from WSRT studies performed by other investigators. We have observed a number of galaxies with the WSRT ourselves, most of them selected such as to achieve an even distribution of well-studied galaxies along the spiral sequence. The results of these 21 cm line observations have been compared with optical information available in the literature, and with recent results from surface photometry of a number of galaxies made by Van der Kruit and Bosma. Since these data have usually been obtained for a different purpose it should be clear that our sample is still limited in several respects.
The structure of this thesis is as follows: In chapter 2 we present the sample of spirals used in our study and we discuss the criteria which lead to the choice of objects we have observed with the WSRT. In chapter 3 we describe various problems associated with the processing of 21 cm HI line data obtained with the WSRT and subsequent interpretation of those data. In chapter 4 we have collected a series of articles on individual galaxies, some of them already published and others ready for submission to Astronomy and Astrophysics. In chapter 5 we present a description of the maps of the HI distribution and the radial velocity fields of the galaxies in the sample. Models of the mass distribution have been determined for 25 galaxies; these are presented in chapter 6. In chapter 7 we compare the results from these models with other physical quantities and try to relate them with morphological type. In chapter 8 we give a brief summary of our results and suggestions for future work.
The main result of our work is that the simple working model for a spiral galaxy which we described above, needs a first order revision. Most spirals do not follow the usual picture of axisymmetric disks in differential rotation with small amplitude perturbations due to spiral arms. In a number of cases large scale symmetric deviations from axial symmetry are found, some of these seem to be associated with oval shaped (oval in the sense of broadly elliptical) structures in the plane of the disk, and others with a warping of the HI layers in the outer parts. Moreover, asymmetries occur in all galaxies, and in some cases these asymmetries have a very large amplitude. The mass models indicate that in the outer parts of a spiral the mass-to-light ratio is higher than in the inner parts. Perhaps a substantial fraction of the mass is not distributed in a disk at all. The ratio of total mass to neutral hydrogen mass tends to remain more or less constant in the outer parts. There is no simple relation between the morphological type of a galaxy and the mass distribution.