The Figure shows the distributions perpendicular to the planes of the galaxies of: (a) the surface density of HI, σHI(R), (b) the surface density of HII regions, σHII(R), and (c) the radio brightness temperature, σTB(R). Details may be found in Berkhuijsen (A.A. 57, 9, 1977; Proc. IAU Symp. 77, 1978); here σHI(R) of the Galaxy was taken from Gordon and Burton (Ap. J. 208, 346, 1976) and σTB(R) of M31 has been revised. Distances to the centre are scaled with the Holmberg radius RH (21, 15 and 7.8 kpc for M31, Galaxy, M33).

The structure and evolution of the Galaxy are reviewed in terms of interactions among various components, including its immediate surroundings. Emphasis is given to the large-scale processes responsible for star formation and chemical evolution, which are seen to be controlled by interactions between the ISM and other components and by gas flows. A model for formation of the Galaxy is outlined, in which the spheroidal component results from mergers of former small galaxies, and the disk has been accreted from outlying diffuse matter.

The galaxy is represented schematically by a three-component model: a disc having the form of a modified exponential distribution, a spheroidal (bulge + nucleus) component and a dark halo component which, following the nomenclature of Einasto, we call the corona. The shapes of these components, chosen on the basis of observations of other galaxies, are consistent with imperfect knowledge of the Galaxy; values of the adjustable parameters are chosen by a least square minimization technique to best fit the most accurate kinematical and dynamical galactic observations. The local radius, circular velocity and escape velocity are found to be (R⊙, V⊙, Vesc) = (9.05 ± 0.33 kpc, 247 ± 13 km/s, 550 ± 24) quite close to the values determined from observations directly. The masses in the three components are (MD, MSp, MC) = (0.78 ± 0.13, 0.81 ± 0.09, 20.3) × 1011 M⊙ for a model with coronal radius of 335 kpc. If the quite uncertain coronal radius is reduced to 100 kpc the model is essentially unchanged except that then MC = 6.65 × 1011 M⊙. The disc and spheroidal components have in either case luminosities (in the visual band of (LD, LSp) = (2.0, 0.2) × 1010 L⊙. The galaxy is a normal giant spiral of type Sb-Sc similar to NGC 4565.

Galactic mass modeling has a long history. The first mass models were designed to represent the galactic attraction force in the radial direction. Considerable progress in galactic mass modeling was made during the fifties when Kuzmin (1952) introduced nonhomogeneous ellipsoids and Schmidt (1956) used a number of ellipsoids to represent various galactic populations. Further progress in galactic mass modeling has followed with the improvement of the system of the galactic constants and with the improvement of our knowledge of the structure of the galactic populations, in particular with the discovery of a massive corona around the Galaxy. In this report we present a new mass model of the Galaxy. It has been constructed using the most recent data available. A preliminary version of this model has been discussed earlier by Einasto, Jôeveer and Kaasik (1976).

In a recent paper, Innanen and Papp (1977) have introduced and discussed the behaviour of a function they called the “super angular momentum,” ht2. This quantity, although not formally an integral of motion in the classical definition, is nevertheless a straightforward generalization of the classical total angular momentum. The super angular momentum, together with the other two classical invariants, the total energy E, and the Z component of the angular momentum hZ lead to the introduction for each homogeneous stellar system of a new collision-free distribution function and fo, β and γ are constants. The function f satisfies the fundamental equation of stellar dynamics and shows that the velocity dispersions of stellar systems cannot be isotropic, but rather that

1. (a) the velocity dispersion in the radial direction is constant (i. e. “isothermal”) and

2. (b) the velocity dispersions in the tangential directions follow the law

We define “metals” as consisting of the elements at and near the Fe-peak, and review several methods by which the metal-abundance gradient of stars in the halo can be obtained. In the inner halo (galactocentric distance R ≲ 8 kpc), the Basel RGU photometry should allow the derivation of the shapes and dimensions of the iso-abundance contours. For the outer halo to R ∼ 30 kpc, we review techniques based on Δs-measurements of RR Lyraes (Lick) and intermediate band-pass photometry of globular-cluster giants (Searle and Zinn, Palomar). Both methods suggest little change in mean [Fe/H] between 10 and 30 kpc; however, both may be biased against the discovery of very metal-poor objects. The conclusion that the outer halo has no abundance gradient may be somewhat premature.

γ-ray data favor a cosmic-ray propagation halo comparable with the thickness of the primary radio emission disk of our Galaxy and the halo seen around NGC891.

An examination of the intensity, energy spectrum, and spatial distribution of the diffuse γ radiation observed by SAS 2 away from the galactic plane in the energy range above 35 MeV has revealed no evidence supporting a cosmic ray halo surrounding the galaxy in the general shape of a sphere. The diffuse γ radiation does consist of two components. One component is related to the galactic disk on the basis of its correlation with the 21-cm measurements, the continuum radio emission, and galactic coordinates. Further its energy spectrum is similar to that in the plane, and its intensity distribution joins smoothly to the intense radiation from the plane. The other component appears isotropic, at least on a coarse scale, and has a steep energy spectrum. The degree of isotropy which has been established for the “isotropic” radiation and the steep energy spectrum, which distinguishes it from the galactic disk radiation, place strong constraints on galactic halo models for the origin of this component. Theoretical models involving a galactic halo have generally postulated a halo with dimensions of the order of the Galaxy and hence a radius, at least in the plane, of about 15 kpcs. Since the Sun is about 10 kpc from the galactic center, if such a halo exists and is responsible for the γ rays (through, for example, black body Compton radiation), a very marked anisotropy would be seen, with the γ ray intensity from the general direction of the galactic center being much larger than that from the same latitudes in the anticenter direction. In fact, no such anisotropy is seen; specifically the ratio of the average intensity in the (300° < ℓ < 60°, 20° < |b| < 40°) region to that in the (100° < ℓ < 250°, 20° < |b| < 40°) region was found to be 1.10±0.19 compared to a calculated value for a model with a uniform cosmic ray sphere with a 15 kpc radius of 2.85. The ratio between the average γ-ray intensity from regions with |b| < 60° to that from 20° < |b| < 40° is found to be 0.87±0.09. If the region is assumed to be spherical, but with a larger radius and a uniform cosmic ray density, the upper limit (2σ) set for the anisotropy demands that the radius be at least 45 kpc. An extragalactic origin for the isotropic component currently appears to be a more plausible explanation.

Cosmic rays were discovered in 1912, but it was only about forty years later that they were found to play an important role in astronomy. Firstly, cosmic rays (including the electron component) are an important source of astronomical information, namely the cosmic synchrotron radiation. Secondly, cosmic rays are essential as energetic and dynamical factors in the galaxy and also as a source of heating and transformation of the interstellar gas composition. Suffice it to remember, for example, that near the solar system the cosmic ray energy density is about the same as the thermal energy of the interstellar gas, and the cosmic ray pressure is likewise about the same as the interstellar gas pressure. Thus, there is every reason to believe that galaxies do not consist of stars and gas only, but of cosmic rays as well.

In 1975 F. J. Kerr and P. F. Bowers of the University of Maryland made a full coverage survey of the neutral hydrogen in the southern hemisphere between ±10° latitude from 240° to 350° in longitude. This survey taken with the CSIRO 18 meter telescope is still in the reduction stage but when completed it will provide an ideal complement to the full ±10° coverage of the northern sky by H. Weaver and D. R. W. Williams (1973). The possibility of a unified analysis of the outer region of the Galaxy has inspired this present study. The plan here is to use the ±10° Weaver-Williams survey in the region 10° < ℓ < 130°; the ±30° extension to this survey (H. Weaver and D. R. W. Williams, 1974) in the region 115° < ℓ < 245° and finally the Kerr-Bowers survey, 240° < ℓ < 350°. Since the latter survey is incompletely reduced at this time we have only used profiles at 5° intervals in longitude and thereby have produced a preliminary determination of the plane in the southern hemisphere. The northern hemisphere determination is complete.

The evidence for warps in the gas layers of galaxies is reviewed. Both the 21-cm line intensity distribution on the sky in edge-on systems and the hydrogen velocity fields in other systems indicate that warped gas layers are common, more common than close companions. Hence, warps probably persist for several times 109 years.

The thickness of the gas layer in NGC 891 appears to increase outward, similarly to that in our Galaxy.

Hunter and Toomre (1969) have demonstrated that a simple warp of a stellar disk will damp out within one or two galactic rotation periods due to rapid differential recession of stellar orbits. If, however, the galactic gravitational field is more nearly spherically symmetric, the rate of differential recession is decreased and the warp may persist for a longer time. We therefore propose that the observed gaseous warps exist in regions outside of the massive stellar disk; that is, in regions where the gravitational field is essentially spherically symmetric. A necessary condition of this hypothesis is that long-lived warps are present only in a low mass, low random velocity component of the galaxy – presumably the gas.

As a theoretical subject, the study of the bending of the gaseous planes of galaxies has not been extremely popular. It seems that this is due to an excellent and thorough paper on the subject, published by Hunter and Toomre in 1969. At that time there existed three (or four) rival theoretical mechanisms:

1. (1) vertical oscillations of the galactic disk, considered first by Lynden-Bell (1965);

2. (2) asymmetrical pressure on the disk due to intergalactic wind (Kahn and Woltjer 1959);

3. (3) tidal influence of the Magellanic Clouds, from

   1. (a) the case of Stationary Clouds, and

   2. (b) the case of Clouds orbiting around the Galaxy (Elwert and Hablick 1965, Avner and King 1967).

The generic label “high-velocity cloud” is a very unspecific term that tells us only that there are clouds of hydrogen gas moving with velocities which are anomalous for their position on the sky, if we assume that we live in a flat, uniformly rotating galaxy. If one attempts to explain them all away in one fell swoop we might draw an analogy with the situation extant in the 19th century, when optical astronomers were aware of patches of emission that were called nebulae. We now know that there are extragalactic as well as galactic nebulae, all quite different from one another. They were recognized to be so different as soon as the quality of data on these objects improved sufficiently. We may be in a similar, somewhat frustrating, position with regard to the high-velocity clouds. Soon after their discovery in 1963 it was believed that the clouds were mostly at negative velocities and that they were very local. Now we know of many more clouds at both positive and negative velocities. The models proposed for their existence cover a range of distances. We still have local (infall and explosive event) models, while other models place the clouds on relatively nearby spiral arms, in the high-z extension or warp areas of distant arms, in the outskirts and immediate neighborhood of the Galaxy, at the distance of the Magellanic Clouds, and even beyond. There may well be some truth in all of these models.

Earlier extended surveys with moderate resolution and sensitivity had shown that high-velocity gas can appear as small clouds (∼2°) either isolated, or in elongated complexes with lengths of up to 30°. The profiles were rather broad (halfwidths of 25 km/s). While the brightest HVC's were found in the northern galactic hemisphere, the highest negative velocities were found in the southern latitudes. The only known high-positive velocity objects were a few at low latitudes between 1 = 245° and 330°.

The galactic anticenter region contains several streams of high-velocity clouds of HI. The principal stream is associated with a localized HI feature at forbidden negative velocities near ℓ, b = 197°, 2° (Weaver 1970, 1974). Here we show that the forbidden-velocity feature, in addition to being the culmination of three streams of high-velocity clouds, is correlated with a disturbance in the permitted-velocity gas. Evidence suggests that this disturbance is located within the Galaxy, implying by association that the anticenter high-velocity clouds and the culmination feature are at distances interior to the Galaxy.

The observational studies of the neutral hydrogen high velocity clouds (HVC's) in the last decade have for the most part concentrated on the detailed study of the prominent complexes discovered by the surveys of the late 1960's. More recently, several HVC's were discovered, often by accident, with very high velocities, which are typically outside of the velocity range and below the sensitivity limit of the old surveys. The tendency to associate these clouds with the closest galaxy at hand became a rather common practice. The idea that the Local Group, or the Local Supercluster, is populated with such “intergalactic clouds” was fed by these discoveries, and generalized to the whole field of the HVC's. I have repeatedly argued against the latter generalization (Giovanelli 1977, 1978). In this paper I shall report on the progress of a new survey that intends to constitute a test of the intergalactic approach itself.

(a) Four high-velocity clouds (HVCs) with large negative velocities have been mapped at high sensitivity, with an angular resolution of 12 arcmin and a velocity resolution of 1.8 km s−1 (Cohen & Mirabel, in press). Although these are small isolated clouds they show the same two-component velocity structure that is observed in the large Northern HVC complexes at lower velocity. In addition the cool dense core of one of the clouds is found to be rotating. This rotation, and also the regular velocity patterns within the four clouds, indicates that the clouds are unlikely to be interacting with galactic material. It is interesting that similar observations of the Magellanic Stream do not show narrow velocity components, suggesting that there is a real physical difference between the Stream and other HVCs (Mirabel, Cohen & Davies, in press).

The Magellanic Stream is an arc of neutral hydrogen which nearly follows a great circle and which contains the Magellanic Clouds - hence its name (Mathewson, Cleary and Murray 1974). This great circle passes within a few degrees of the south galactic pole and lies close to the supergalactic plane. Mathewson and Schwarz (1976) argued that this indicates that the Magellanic Stream and Magellanic Clouds are not bound to the Galaxy. To reinforce this argument, they pointed out that around the supergalactic plane there is a similar systematic variation in the velocities of the Local Group and those of the Stream which may be due to the reflection of the motion of the galactic center if the velocity of rotatior of the Sun is 225 km s−1; if it is 290 km s−1 then the grounds for this argument would disappear.

The tidal and the primordial theories for the Magellanic Stream are examined in a frame of test-particle simulation for the interacting triple system of the Galaxy, the Large and Small Magellanic Clouds (LMC and SMC). Difficulties of the radial velocity of the Stream still beset these two theories. Several new models for the Stream and the Clouds are briefly discussed in relation to the bending of the galactic disk, the past binary orbits of the LMC and SMC and also the Local Group and the Local Supercluster of galaxies.