The distribution of light in galaxies is their most obvious and fundamental observable property. Hopefully it gives us some insight into their structure and dynamics. In this talk I will review some recent work on ellipticals and disk galaxies. In summary, the luminosity distributions for both these classes have several complexities, the dynamical significance of which is not yet clear. Both classes turn out to have roughly constant mean surface brightness. This may result from selection. However, if it is real, then it is important dynamically, and I will discuss this question at the end.

21cm observations at the Arecibo Observatory for 9 edge-on spiral galaxies are described. Flat rotation curves are found in most cases.

This review concerns the large-scale structure of radio continuum emission in spiral galaxies (“the smooth background”), by which we mean the distribution of radio surface brightness at scales larger than, say, 1 kpc. Accordingly the nuclear emission and structure due to spiral arms and HII regions will not be a major topic of discussion here. Already the first mappings of the galactic background suggested that there is indeed a distribution of radio continuum emission extending throughout the Galaxy. This conclusion has been reinforced by the earliest observations of M31 by showing that the general emission from this object extended over at least the whole optical image. More recently, van der Kruit (1973a, b, c) separated the radio emission from a sample of spiral galaxies observed at 1415 MHz with the Westerbork Synthesis Radio Telescope (WSRT) into a nuclear, spiral arm and “base disk” component, showing that the latter component usually contains most of the flux density. This latter component is largely non-thermal and extends over the whole optical image (see also van der Kruit and Allen, 1976). Clearly it is astrophysically interesting to discuss the large-scale structure of the radio continuum emission.

If we look at the radio properties of the nearby ellipticals we find a situation considerably different from that just described by van der Kruit for the spiral galaxies. For example NGC 5128 (Cen A), the nearest giant elliptical galaxy, is a thousand times more powerful a radio source than the brightest spiral galaxies and furthermore its radio emission comes from a multiple lobed radio structure which bears no resemblance to the optical light distribution (e.g. Ekers, 1975). The other radio emitting elliptical galaxies in our neighbourhood, NGC 1316 (Fornax A), IC 4296 (1333–33), have similar morphology. A question which then arises is whether at lower levels we can detect radio emission coming from the optical image of the elliptical galaxies and which may be more closely related to the kind of emission seen in the spiral galaxies.

I have been asked to review the physical principles which underlie the dynamical equilibrium and stability of a composite system of gas, magnetic field, and cosmic rays. What is of particular concern here are those aspects which control the distribution of magnetic field and cosmic rays, and thus influence the morphology of galaxies as seen in nonthermal radio emission.

The past decade has witnessed dramatic changes both in our conceptual models of disk-system formation and evolution and in the power of new observational techniques to confront, challenge, and redefine these models. In this contribution, we would like to review recent optical wavelength studies of spiral and SO galaxies which appear to influence our understanding of disk-system evolution. Particular emphasis will be placed on the effects of environment on evolutionary processes, since it appears likely that the addition or removal of gas during the lifetime of a disk system may often be dominant in controlling its appearance.

If we view spiral structure in galaxies as a manifestation of some dynamical process which we wish to understand, then the observer can distinguish two facets of the problems. In the first place he must consider the observable consequences suggested by theoretical developments. In the second place he must endeavour to interpret the available observations in terms of realistic physical models, where such models may require detailed information beyond that required for an understanding of spiral structure as a dynamical phenomenon. We shall consider these two aspects in turn.

The spiral galaxy M81 is a challenging object for testing a density-wave theory. It is of large angular size and favourably inclined to the line of sight; radio observations of neutral atomic hydrogen showing well defined spiral arms with noncircular motions are available (Rots and Shane 1975), and there is evidence from surface photometry for density waves in the old disk population (Schweizer 1976). Can these observations be unified in one density-wave model?

It would be a mistake to think that the density wave theories of spiral structure have reached the maturity where they can make unconditional predictions which can be tested. On the contrary, they are still very dependent on observations for help and guidance.

The observation in 1965 of absorption due to the 18 cm ground-state transition of OH in the direction of Cas A (Weinreb et al., 1963) marked the first occasion on which a molecular cloud was detected at radio wavelengths. However, it was not until the later discovery of high-intensity OH emission (Weaver et al., 1965) that attention turned to nearby galaxies. Unfortunately, searches for OH emission in the Magellanic Clouds (McGee et al., 1965; Radhakrishnan, 1967) and in more distant galaxies (Roberts, 1967) were unsuccessful. The first detection of an OH transition, in absorption against the radio continuum of NGC 253 and 3034 (Weliachew, 1971), went almost unnoticed because the results were unconvincing. However, Whiteoak and Gardner (1973) and Nguyen-Q-Rieu et al. (1976) confirmed the existence of the absorption.

Recent satellite detections of 0 VI absorption and soft X-ray emission leave little doubt that, locally, a substantial fraction of the volume of space between interstellar clouds must be filled with rarified and highly ionized gas at temperatures ranging from 2 × 105 to > 106 K. (See the reviews of Spitzer and Jenkins 1975, and Kraushaar 1977.) The physical state of this gas contrasts sharply with the theoretical picture of a largely neutral, warm, intercloud medium at ∼ 104 K developed by Pikel'ner (1967), Field, Goldsmith, and Habing (1969), and Spitzer and Scott (1969). My purpose here is to review the evidence, observational and theoretical, concerning how extensive hot gas at ∼ 106 K might be.

The two nearest spiral galaxies, M31 and M33, have been extensively studied both at optical as well as at radio wavelengths. Reviews of radio continuum observations are given in van der Kruit and Allen (1976) and in von Kap-herr et al. (1978), respectively. In Table 1 new radio observations in various stages of progress are listed, all of which show — or are expected to show — spiral structure. In this paper the Effelsberg maps at 11 cm of M31 and at 6 cm of M33 are discussed and compared with optical data. Some attention is given to the separation of thermal and nonthermal emission which is essential to any discussion of the origin of the radiation.

1) Photometry from de Vaucouleurs 1958 shows a bulge obeying the r1/4 law up to less than 5 pc from the center with an effective radius re = 17′5. In the region from 0.01 to 0.2 re, equal luminosity curves are well approximated by similar ellipses of axial ratio 0.68. The reduced spatial density υ* and the reduced gravitationnal potential φ* (i.e. for a mass to luminosity ratio f equal to 1) can then be easily computed (Monnet and Simien 1977).

The H I, H II, and OB-association densities, as a function of radius, have the same structure, reminding one of a ring with a maximum at r ≃ 50′.