Lunar occultation observations of the thermal sources Sgr B2, G0.9 + 0.1 and G1.1-0.1 at 327 MHz have been used to estimate their electron densities and temperatures. A new nonthermal source of size ∼ 10 × 5′ has been found about 7′ to the south of G1.1 − 0.1. A brightness contour diagram with a resolution of approximately 25 × 6′ is presented for the background radio emission near the sources Sgr A and Sgr B2.

Features of the 21-cm line radiation that have often been attributed to the galactic center or to activity there include, near the equator, the ‘3-kpc arm’, the ‘expanding arms’ at +70 and + 135 km s+1, and the ‘nuclear disk’, and, away from the equator, features observed by Cugnon, by Shane, by van der Kruit, and by Sanders, Wrixon, and Penzias, among others. Models of the features in the equatorial plane are discussed on the basis of the form of the Tb(l, V) contours that they generate. Both explosions in the nucleus and resonance with a spiral wave have been suggested as the cause of the 3-kpc arm and related features. Some of the models put limits on the mass distribution, particularly the amount of condensed mass at the center. Higher-latitude features have been less thoroughly modeled, but explosions in the nucleus and resonant orbits have also been suggested for their origins.

Dense gas clouds containing OH, CO, NH3 and H2CO are found in the inner part of the H1 nuclear disk. The molecular spectral lines allow direct observations of the kinematics of the gas near the galactic centre. Strong absorption of the thermal continuum sources by OH and H2CO shows that much of the gas on the near side of the centre can be located in a massive ‘ring’ expanding at 130 km s-1 which may have originated close to the nucleus about 106yr ago. Observations of CO emission from beyond the centre show that the far side of the ‘ring’ is expanding at a lower velocity, less than 90 km s-1. Observations of CO and NH3 emission with positive velocities for l < 360° are needed to establish whether the ‘ring’ is a continuous structure.

OH and H2CO are also observed to be falling towards the centre. There is no agreement as to the location of this infalling matter.

The nuclear regions of the Galaxy are compared with those of NGC 253, particularly in regard to expansional velocities, IR and radio emission, and OH absorption.

The J = 6 to 5 line of OCS at 72.9768 GHz was detected in Sgr B2 with antenna temperature 0.3 K, velocity + 62 km s-1, and velocity half width of ∼ 25 km s-1.

The phenomena displayed by the interstellar medium in the galactic centre are considered. The asymmetries shown by the features between 1 and 3 kpc from the centre together with the presence of material lying out of the galactic plane favour the expulsion hypothesis for their origin. The nuclear disk shows a perturbation which might have resulted from such expulsion. The dense molecular clouds in the disk may well be considered as the most direct evidence that matter is expelled from the nucleus and that this occurs at a high rate. The +50 km s-1 feature in the direction of Sgr A may be the most recently expelled body of molecular gas. New observations of the central radio source, Sgr A, have revealed details on a very small scale, and the infrared core also shows a complicated structure. Probably a number of individual concentrations of gas and dust are present. While the position of the actual nucleus seems now to have been defined to within a few arcseconds, no indication has yet been found concerning its nature nor concerning the mechanism that enables it to expel the vast expanding masses of gas observed in the central region.

A number of current problems in 21 cm line studies of the Galaxy as a whole are discussed. Because of the difficulties involved with straightforward mapping, it is important to isolate integrated and other properties of the hydrogen profiles, the interpretation of which does not require accurate distance determinations. In addition, methods of analysis are necessary which either account for or exploit the sensitivity of hydrogen profiles to velocity irregularities and to geometrical configurations. The model-fitting approach to the interpretation of the hydrogen profiles is useful in this respect. Extragalactic hydrogen studies which show the relative ordering of the various components of spiral structure can inspire research in our own Galaxy. Such investigations are necessary for an understanding of the forces governing the spiral structure. It seems that the neutral hydrogen is primarily a tracer of locations where the overall distribution of stars is producing a gravitational sink. Other spiral tracers, in particular the molecules, are better considered as tracers of regions where the gas has been compressed, perhaps (at least on a large scale) by the shock front predicted by the density-wave theory.

The principal problem in the study of spiral structure is to reduce the data to a small enough number of pictures or numbers that can be easily comprehended. A new map has been produced, based on circular rotation, but the high probability of large radial motions makes it likely than any picture of spiral structure derived on current theories will be in error. New computer pictures have made it possible to see the structure of the outer arm, which has many cloud-like structures lying above it on one side, some reaching heights of more than 1 kpc. Some very weak extended features in the outer regions are described, and examples of holes and jets are discussed.

The survey has been completely re-observed using the resurfaced NRAO 300-ft telescope. It covers the latitude ranges b = − 1.5° to + 1.6° from l = 11° to 60°, b = − 2.2° to + 2.4° from l=60° to 132°, and b = − 1.9° to + 2.4° from l = 132° to 235°. Both the new survey and the old survey are available on magnetic tape and 70-mm film.

The rolling motion phenomenon is interpreted as an apparent motion in the observations. It is caused by (i) the combined effect of the differential rotation and the bending of the galactic plane for the spiral arms beyond the solar circle, or (ii) the displacement of the spiral arms from the central galactic plane for those within the solar circle.

A review is given of the observations of neutral hydrogen high velocity clouds (|ν| > 80 km s−1) in and near the Galaxy. The positive and negative clouds are seen to have different distributions in the sky, following roughly the velocity pattern of galactic rotation. A characteristic of the majority of the clouds is their distribution in elongated bands or strings. The various theories of origin of HVCs are discussed; the possible role of the tidal interactions between the Magellanic Clouds and the Galaxy is emphasized. Tests are suggested to distinguish between the Oort theory of the infall of intergalactic material and theories which envisage the HVCs as originating in the outermost spiral structure.

A southern sky survey of Hiin the velocity range − 340 km s−1 to + 380 km s−1 has shown that a long filament of H iextends from the Small Magellanic Cloud (SMC) region down to the South Galactic Pole and connects with the long Hifilament discovered recently by Wannier and Wrixon (1972) and van Kuilenberg (1972). There is also some evidence that this continues on the other side of the Magellanic Clouds and crosses the galactic plane at l = 306°. This filament, which follows very closely a great circle over its entire 180° arc across the sky, is given the name ‘The Magellanic Stream’. It may have been produced by gravitational interaction between the SMC and the Galaxy during a close passage (20 kpc) of the SMC some 5 × 108 yr ago, although it is impossible to account for the observed radial velocities along the Stream unless some force other than gravity is invoked to act on the Stream as well.

The correlation of Hiand dust at high latitudes gives different results depending on whether one uses galaxy or reddening data for the dust distribution. It is not certain to what extent this depends on differences in the gas to dust ratio or on difficulties in determining the amount of dust from the optical data. The correlation of Hiand continuum radiation shows Histructures associated with the large-scale loops. There is also a coincidence of a hole in both the Hiand continuum.

Low- and intermediate-velocity Gaussian components of hydrogen emission profiles observed at positive galactic latitudes are investigated. The low-velocity hole, as called by Wesselius and Fejes (1973), is a phenomenon which appears in the layer composed of the gas emitting narrow Gaussian components but does not in that of wide components. Both Models I and II suggested by Wesselius and Fejes contradict this result. A hypothesis is proposed that a stream motion in the layer emitting wide Gaussian components, which may be identified with the intercloud medium, swept away the interstellar clouds emitting narrow Gaussian components in the region of the hole.

Recent studies of high latitude brightness have shown that in several regions a significant proportion of it is due to loop features. The largest of these are now thought to be local (within a few hundred parsecs) and perhaps to be due to supernova events.

Aperture synthesis maps of 45 external spiral galaxies have shown large-scale continuum nonthermal features in the disks of some and, in general, an absence of spherical halos around the galaxies. This supports the view that the radio emission from our Galaxy can best be described as coming from a thick disk. Any large-scale spherical component could not have a volume emissivity greater than 1.5% of the average volume emissivity of the disk at meter wavelengths.

A comparison of the disk radiation with new disk models suggests that the galactic plane radiation has two components: a base disk and a spiral component. Each of these contributes ~50% of the total power output of the disk at 150 MHz.

The 100 m telescope was used to complete the northern sky survey started at Jodrell Bank. The survey aimed at reaching the confusion limit of the telescope. Zero levels are consistent within ± 3 K, and scale errors are less than 10%. The survey will be published in the form of maps; a machine readable version is also available.