A total of 9 nonthermal sources near the plane of the Galaxy have now been definitely identified with nebulosities. The identifications are well established; positions and sizes of the nebulosities and of the sources are in satisfactory agreement. At least three types of objects can be recognized:

1. Remnants of supernovae of type I.

2. The Cassiopeia A source, a remnant of a supernova of type II.

3. The Cygnus loop and several other large nebulosities which are most likely of similar origin. They also may be remnants of supernovae type II.

The large size and the low surface brightness of these objects render their detailed investigation a difficult and time-consuming task. The true picture may emerge slowly, and conclusions drawn at an early stage of the work may have to be discarded as the investigation progresses.

Within our Galaxy there are a number of nonthermal sources of radio emission. In the last few years a considerable amount of data has been collected about some of these sources, in particular the Crab nebula and the Cassiopeia A source. There is strong evidence now to suggest that all of the nonthermal radio sources emit by synchrotron radiation, or acceleration radiation as we shall describe it here. In this paper we want to discuss the physical conditions in two objects, the Cygnus loop and IC 443.

Several extragalactic radio sources within reach of Sydney radio telescopes have been found to have angular sizes so great that they can be resolved with the pencil-beam aerials, which are available at three frequencies. It is possible to study in some cases the brightness distribution in some detail; in others to study the qualitative general picture. The observations with which the present paper is concerned are set out in Table I.

Observations of 17 intense sources were made at a 1.9-meter wavelength with the Cambridge four-aerial interferometer [1] to provide information that may assist in their identification with optical objects, and as a check on the positions of a few of the sources of the recently completed third Cambridge survey [2].

The radio source Cygnus X (IAU 20N4A) has long created interest because of its unusual spectrum, its seemingly extended nature, and its location in a region where we believe we look nearly lengthwise down a spiral arm of the Galaxy. Piddington and Minnett [1] first showed that the source had a thermal spectrum. In a recent paper Davies [2] has concluded that the source is an extremely large extended HII region located in the second spiral arm in Cygnus. The recent construction of large parabolic antennas has made it possible to clarify greatly the nature of Cygnus X; it is the purpose of this paper to give the results of a high-resolution survey of the Cygnus X region, and to report a possibly relevant optical observation.

Kraus and Ko [1] and Hanbury Brown and Hazard [2] have suggested that the band of radio emission running roughly perpendicular to the galactic plane at about 12h right ascension, represents the integrated emission from the concentration of bright galaxies lying along a great circle that crosses the galactic plane at longitudes l = 105 degrees and 285 degrees. This paper puts forward certain difficulties in this interpretation. These galaxies and our own are believed by de Vaucouleurs [3, 4] to be members of a “cluster of clusters,” which he terms the local supergalaxy, after Shapley.

The previous unsuccessful attempts to detect the 327-Mc/s line of galactic deuterium [1, 2] were limited in sensitivity either by the size of aerial or by the inability to use long integration times with a large fixed aerial. The new observations reported here were made with the 250-foot radio telescope at Jodrell Bank, which can continuously follow a source of radiation; moreover, the effective integration time was improved by using a multi-channel receiver instead of a swept-frequency one.

Detailed structure within the spiral arms of our Galaxy is suggested by hydrogen-line spectra taken with high resolution in frequency [1]. The spectra show much detail in each maximum (spiral arm). It is not clear, however, if this frequency structure refers to fine structure in depth or in velocity dispersion or in both. Fine structure in position and depth has been inferred from 21-cm drift curves taken across the nearby spiral arms. The results of three investigations will be discussed. Two have been published in some detail [2, 3] and will only be summarized here.

This paper is a short survey of the results obtained with the Dwingeloo 25-meter radio telescope (beamwidth 0°.56) and the 21-cm receiver on hydrogen-line absorption effects in the spectra of a number of strong point sources. Earlier work on this subject has been done by Hagen, Lilley, and McClain [1], and by Davies and Williams [2]. Some preliminary results of the first part of the present investigation were already published [3].

Observations of the neutral hydrogen content of the galactic clusters h and χ Persei, the Pleiades, the cluster in Coma Berenices, Praesepe, and M 67 have been made with the Harvard College Observatory 60-foot radio telescope. The receiver used was the original receiver made by the Ewen Knight Corporation for the Harvard 24-foot radio telescope. The observations were of three kinds: (1) right-ascension drift curves at 50 kc/s bandwidth, with the receiver fixed-tuned to the frequency of maximum cluster emission, if known, or to the frequency given by the cluster optical radial velocity; (2) conventional frequency profiles at the position of maximum cluster radiation; (3) “residual” frequency profiles, where there is significant general galactic radiation at the cluster frequency and position. These data provide the spatial distribution of any H i in the cluster vicinity that is in excess of the general galactic H i, and a frequency profile for this excess hydrogen. Any excess H i found is considered associated with the cluster if it agrees in velocity and position with the optical cluster.

Neutral hydrogen is the main constituent of interstellar material. While solid particles and ionized hydrogen have long been studied, detailed investigations of the distribution of neutral hydrogen have only recently become possible. The first survey of neutral hydrogen in Orion was made by Menon [1, 2] with the Harvard 24–foot telescope and reported at Jodrell Bank in 1955. A more detailed study of the structure and dynamics of the interstellar medium in this region is now in progress with the 25–meter reflector at Dwingeloo. Its beamwidth is 0°.57 at 21 cm, corresponding to 5 parsecs at the distance of the Orion association. The bandwidth of the receiver was 20 kc/s, or 4 km/second, in this program. Observations with 10 kc/s bandwidth are being added.

Using an 8-meter Würzburg parabola on an equatorial mount in Washington, D. C., we have completed about half of the necessary observations and reductions for a survey of the 1420-Mc/s hydrogen emission for all sky areas near the galactic plane which are visible here.

Approximately 1000 observations of neutral hydrogen have been obtained with the 54-channel H-line receiver and the Würzburg antenna of the Carnegie Institution of Washington. H-line profiles have been observed at 10-degree intervals along the ±20-, ±30-, and ± 40-degree parallels of galactic latitude; at 20-degree intervals along the ± 50- and ±60-degree parallels; at 40-degree intervals along the ±70- and ±80-degree parallels and at the poles. Approximately two dozen observations have been taken at points near the galactic plane in order to correlate these observations with the Leiden survey [1]. The beamwidth of the Würzburg antenna was about 2 degrees. The observations were taken in two series, one series during the summer of 1957, and the other series during 1958 January. The video frequency bandwidth of the receiver is 12 kc/s. The profiles consist of averages of from two to six scans with integration times from 4.8 to 7.5 minutes.

This session contained all the papers that did not fit very well in the other sessions, i.e., those papers dealing neither with the solar system nor directly with problems of galactic structure nor with the faint sources and their statistics. This negative criterion has led to a variety of contributions, the common denominator of which is that we try to find out as much as we can about everything. I shall try to summarize the main points and add my own comments.

The subject matter of this session includes both 21-cm and continuum evidence on large-scale features, i.e. features ranging in size from spiral arms to whole galaxies, of our own and of external galaxies. Because the individual papers deal with only some of the separate facets of the subject it is desirable to outline the main lines of investigation and to indicate their interrelations and their physical significance.

The results of the survey of the galactic system in the hydrogen line made in the Netherlands have been extensively published and have been discussed at the symposium in Manchester and the I.A.U. assembly in Dublin. I shall not enter into a renewed discussion of these, but rather consider what has been added since.

While trying to determine the rotation curve in 1953 Kwee, Muller, and Westerhout [1] found long and faint wings in the line profiles within 20 to 25 degrees longitude from the center. The gas responsible for these wings should therefore lie within 3 kiloparsecs from the center. The velocities of the neutral hydrogen causing these wings are far greater than could be expected on the basis of a reasonable rotational model of the Galaxy. Therefore, the wings were tentatively interpreted as being caused by high turbulent velocities in the gas. In view of the new and better data obtained with the 25-meter telescope in Dwingeloo, this interpretation has now been dropped. The new conclusion is that all of the neutral hydrogen in these regions is expanding and at the same time taking part in the galactic rotation. The evidence for this conclusion will be briefly presented.

In the past two years several extragalactic systems were observed for 21-cm emission with the 25-meter telescope at Dwingeloo. The results on the Andromeda nebula (mainly points along the major axis) have already been published [1]. Observations of M 33 and M 101 are finished, a few other systems are still being observed.

The study of meter-wave radiation from the Milky Way has been hampered, until recently, by the difficulty of obtaining sufficiently high resolution. Observations using interferometers had early indicated the probability of fine structure in the radiation distribution, but the interpretation of these observations was not unique. It was not until the Sydney 3.5-m cross-type radio telescope (beamwidth 50 minutes of arc) was put into operation in 1954 that it became possible to study the distribution of meter wave emission in detail. The observational program with this instrument, together with most of the data analysis relating to the galactic emission, has now been completed and some of the results have already been described [1, 2]. It seems appropriate at this time to give a rather detailed account of the results of this completed work and to discuss its astronomical and astrophysical significance.

Continuum surveys were made at 75 cm (400 Mc/s) and 22 cm (1390 Mc/s) with the Dwingeloo 25-m telescope.