Estimating the forces in the z-direction that affect the disks of spiral galaxies in reasonable galactic mass models we find (Rohlfs and Kreitschmann 1981) that all external forces are small compared to the self-gravity of the disk so that Spitzer's (1942) self-consistent sheet model should give a good description for the z-distribution of the disk where it is well visible. But then the three parameters describing this shape are connected by the formula

One of the most controversial astronomical paradoxes seems to be the mass paradox : the mass of large astronomical systems depends on the mass determination method. This paradox can be explained by supposing the existence of huge invisible coronas of unknown origin around galaxies. In the present paper we discuss basic empirical evidence of hidden matter in galaxies and examine the total mass distribution.

Two dwarf spheroidal galaxies are associated with the Magellanic Stream with their major axes oriented along it. Evidence suggests that all the dwarf spheroidal satellites of the Galaxy belong to one or other of two streams of tidal debris. If this is true the orbits will give the first reliable determination of the total mass of the Galaxy out to 120 kpc.

As a continuation of our earlier work (Gottesman and Hunter, 1982), we have reobserved the HI emission from the galaxy NGC 3992. We have combined all the data and produced new maps, at a significantly improved signal-to-noise ratio, of the gas density and velocity distribution with resolutions of ~ 22″ and 25 km s−1. The resultant, angle averaged, HI rotational velocity is shown in Figure 1 for the symmetric and nearly circular flows for r ≤ 3.35′ from the center (r ≤ 14.0 kpc, assuming de Vaucouleurs' (1979) distance of 14.2 mpc for NGC 3992). Shown, also, in Figure 1 is a fit to the observations provided by a Toomre disk of index n = o. No attempt was made to fit the observational data within 1′, in view of the low signal to noise.

The existence of a massive dark component to the matter distribution of galaxies (the ‘missing mass’) is inferred from the now overwhelming evidence for flat rotation curves in galaxies. However observational data on the linear extent of such a dark component and its total mass contribution is usually restricted by the limited radial distance to which rotation curves of individual galaxies can be measured (typically < 100 kpc). The magnitude of the mass contained within a larger radius around a galaxy can in principal be inferred by studying the kinematics of small groups of galaxies and making assumptions about their dynamical stability (see Faber and Gallagher, 1979, for review). However, one of the major difficulties in such studies is the question of group membership. The inclusion of disrelated foreground or background galaxies into a dynamical calculation of mass obtained for example via the Virial Theorem, can lead to spurious results. The effects of varying membership criteria on the dynamical properties of groups is well illustrated by the work of Huchra and Geller (1982).

Recent evidence for the tidal interaction among neighboring galaxies in the form of HI bridges and streamers has been accumulated by Haynes (1981). The successful application of computer simulations to model these encounters has largely been restricted to the nearby systems such as the Milky Way. In this paper, I will report on preliminary observations of the HI distributions associated with two interacting systems - VV 371 and VV 329.

We report the results of HI observations of eleven gas-rich SO/SOa galaxies with the Westerbork Synthesis Telescope. The majority of these galaxies have most of their hydrogen outside the optical body, in annular configurations with diameters ~2 times the optical. These outer gas rings are often clumped, incomplete, and in approximately circular motion. They may represent the remnants of primordial, often warped, gas disks; or they may have formed from gas (or dwarf galaxies) accreted recently. Optical spectra could discriminate between these possibilities.

A few objects have filled gas disks, and several have inner HI rings with radii ~0.4 × optical. Two objects have peculiar distributions suggesting, respectively, tidal effects and stripping. However, stripping by intergalactic gas appears not to be a major current process.

Synthesis maps of HI in three SO galaxies are discussed. In one case several companions are observed. An evolutionary sequence based on accretion is suggested.

Spiral structure is associated with a slow redistribution of gas, which may already be quite significant over time scales short compared to the age of the galaxy. One has to worry about replacing the gas in order to keep the structure alive.

The hypothesis of quasi-stationary spiral structure in galaxies was explicitly formulated in the early 1960's in papers of Bertil Lindblad and of Lin and Shu. It asserts that the grand design observed in spiral galaxies may be described by the superposition (and interaction) of a small number of spiral modes. (See Lin and Bertin, 1981 for a fairly extensive review of the theory.) We wish to re-affirm the correctness of this hypothesis in the present contribution. Early numerical experiments by P.O. Lindblad and by Miller, Prendergast and Quirk demonstrated that spiral structures occur naturally in certain models of stellar systems, although it was difficult to control the morphological types of galaxies simulated. We are now able to simulate galaxies of various morphological types in a controllable manner. Numerical fluid-dynamical codes developed by Pannatoni (1979) and improved by Haass (1982) have been used to calculate normal modes of various spiral types (Haass, Bertin, and Lin 1982) in the morphological classification of Hubble, Sandage, and de Vaucoulers. Furthermore, the processes that govern the maintenance and the excitation of these modes simulating both normal spirals and barred spirals, can be understood by using analytical theories which are closely related to the local dispersion relationship, as Bertin will describe in his paper at this conference. Understanding these mechanisms enables us to choose the parameters and the distribution functions in our models more properly in order to exhibit the desired characteristics in the computed modes. Such an approach also has important implications on observational studies. Much of the previous work on comparison between theory and observations in normal spirals used only the short trailing waves. A mode must consist of at least two waves propagating in opposite directions. It has been found that, at least in normal spiral modes, the long wave branch provides essentially only a modulation of the amplitude along the short wave branch, which accurately describes the phase. Previous calculations are thereby justified. [These points were not adequately covered in the previous paper reviewing theory of spiral modes.]

Progress in understanding physical mechanisms for the excitation and maintenance of spiral structure has considerably benefited from investigations of tightly wound spiral density waves (e.g., see Bertin 1980). These studies have identified the existence of four basic kinds of density waves (trailing and leading waves, and in each case short and long waves) with different propagation properties. In addition, they have led to the conclusion that some realistic galaxy models can support self-excited global normal spiral modes. These owe their maintenance to the presence of trailing waves with opposite propagation properties and are excited mostly as a result of a WASER (superreflection) mechanism at corotation. In discussing the dynamics of spiral structure and in comparing theory with observations a number of important issues should be kept in mind (Lin and Bertin 1981). Here we just recall that the calculation of spiral modes is being pursued by many researchers, using different methods. In general the structure and the growth rates of the dominant modes are determined by the radial distributions of the active disk density, the differential rotation, and the dispersion speed through the dimensionless functions εo, j, and Q (Haass, Bertin and Lin 1982).

A simple two-component fluid model of a galaxy is analyzed numerically. For this equilibrium configuration a large number of unstable spiral modes is found. It is of particular interest that some of these modes are well described by the asymptotic theory developed for tightly wound trailing spirals, while others are best understood in terms of the swing formalism which includes both leading and trailing waves.

The global modal analysis is applied to a family of models which are consistent with recent dynamical data of NGC 3115. A halo component at least as massive as the disk component is required to explain the apparent lack of spiral structures in NGC 3115.

Many simulations of disc galaxies exhibit a succession of transient spiral patterns when sufficient halo is included to suppress the global bar instability (Hohl 1970, Hockney and Brownrigg 1974, Sellwood and James 1979). This behaviour causes the velocity dispersion to rise secularly which in turn reduces the intensity of the patterns. Spiral activity usually ceases altogether as Q tends asymptotically to a value somewhat greater than 2.

Numerical experiments have shown that axisymmetric shearing gas disks will develop a spiral pattern when a non axi-symmetric contribution is introduced. This mechanism initially seems to be a prime candidate as the source of galactic spiral arms. The experiments have, however, failed to explain the formation of more than the most open systems and the smooth transition in the morphological sequence does not indicate any change in the physical mechanism between Sa and Sc types.

We study the gas in a spiral galaxy with a cloud-dominated “stellar association-perturbed” interstellar medium from the standpoint of a cloud-particle model. Through N-body computational simulations, we follow the time evolution of the system of gas clouds and the corresponding system of young stellar associations forming from the clouds. Basic physical processes are modeled in a three-step cyclic procedure: (1) dynamical propagation of the clouds and young stellar associations, (2) simulation of cloud-cloud collisions, and (3) formation of new associations of protostars that are triggered by the local mechanisms of cloud-cloud collisions and cloud interactions with existing young stellar associations.

Bash and Peters (1976) suggested that giant molecular clouds (GMC's) can be viewed as ballistic particles launched from the two-armed spiral-shock (TASS) wave with orbits influenced only by the overall galactic gravitational potential perturbed by the spiral gravitational potential in the arms. For GMC's in the Milky Way, the model predicts that the radial velocity observed from the Sun increases with age (time since launch). We showed that the terminal velocity of CO observed from l ≃ 30° to l ≃ 60° can be understood if all GMC's are born in the spiral pattern given by Yuan (1969) and live 30 × 106 yrs. Older GMC's were predicted to have radial velocities which exceed observed terminal velocities.

We derive the radial distribution of the specific angular momentum j=J/M, for the gas in M31, M51 and the galaxy, objects for which well observed unsmoothed rotation curves are available in the literature. We find the specific angular momentum to be anti-correlated with the present stellar formation rate, i.e. minima of spin angular momentum correspond to the loci of spiral arms. We find that the stellar formation rate is an inverse function of j. We derive new values of Oort's A constant for the arm and interarm regions in the solar neighborhood.

One main characteristic of the ionized and neutral component of the interstellar medium in galaxies is the appearance of shells and bubbles in all sizes up to 1 kpc diameter (Hodge, 1974; Heiles, 1979). Between different galaxy types differences in size and radial distribution of these bubbles are observed as a function of galaxy type. The stochastic selfpropagating star formation model is able to simulate such distributions on a global galactic scale.

Close inspection of the channel maps which make up the high resolution M31 HI-data set (Brinks, this volume) resulted in finding numerous shell structures and bubble-like regions. To avoid any bias in the interpretation we shall call holes, in the course of this contribution, all those structures where we find a deficiency of neutral gas. Some of the holes can be identified directly in a total surface density map of M31, but due to blending of the various velocity components, e.g. the warp, holes tend to loose contrast. Therefore the best way to search for the holes is by inspection of the full resolution channel maps (δα x δδ x δV = 24″ x 36″ x 8.2 km s−1).