The subject of the 21-cm. line needs no introduction to this audience. Perhaps as an introduction I may mention one moment that belongs to the pre-history of the investigations of this line. In the spring of 1944 Oort said to me: ‘We should have a colloquium on the paper by Reber; would you like to study it? And, by the way, radio astronomy can really become very important if there were at least one line in the radio spectrum. Then we can use the method of differential galactic rotation as we do in optical astronomy.’

The primary project in the current 21-cm. hydrogen-line work in Sydney is a study of the southern Milky Way by F. J. Kerr, J. V. Hindman, and the author. Earlier observations* made at the Radiophysics Laboratory during 1954 yielded a tentative picture of spiral structure in the southern portion of the conventional galactic plane. That picture, slightly modified by more recent results, is shown in Fig. 1.

(1) The suggestion of systematic deviations from circular motion by an angle ϕ, constant throughout the Galaxy, was first presented (Edmondson, 1955) [1] as an ad hoc way to recompute distances so as to make the long circular arm discovered by the Leiden observers (van de Hulst, Muller and Oort, 1954) [2] spiral in. The logarithmic spiral motions postulated in this model are not necessarily along the spiral arms, in contrast to the assumption made by Mrs Rubin (1955) [3].

From November 1953 to August 1955, about 2500 line profiles have been measured with the Kootwijk receiver, under the supervision of Ir C. A. Muller. His receiver is of the well-known frequency-switching type. Two pairs of channels in the second i.f. amplifier, about 5 Mc./s., have band-widths of 36 kc./s. The components of one pair are 1080 kc./s. apart; the second pair is shifted 500 kc./s. with respect to the first. Each pair of channels, combined with the continuously variable second local oscillator and with the fixed, crystal-controlled pair of first local oscillators, which are also 1080 kc./s. apart, gives a switching system. The power at a certain frequency is compared with that at a 1080 kc./s. higher frequency during half the switching period and with that at a 1080 kc./s. lower frequency during the other half by switching between the two first local oscillators. The signals are fed through an integrating network with a time constant of 54 sec. At the outputs of the two pairs are two separate recorders, each giving one-half (or more) of a line profile (see Fig. 1). A continuous frequency calibration of the second local oscillator provides 10 kc./s. markers on the records, accurate to 1 in 106. The limiting sensitivity is 0·7°K., with an overall noise figure of 6·3 (8·0 db.). The 7·5 m. Würzburg aerial has a beam-width between half-power points of 1°·9 × 2°·7.

In a strip 20° wide around the galactic equator, extending from l = 340° to l = 220°, line profiles were measured at 620 points. The interval in latitude was 2°·5, the longitude interval was 2°·5 from l = 340° to 110°, and 5° from l = 115° to 215°. At some longitudes the latitude range was extended to ±.

The determination of the distribution of hydrogen from 21-cm. observations in parts of the Galaxy, which are nearer to the centre than the sun, is seriously handicapped by the fact that the observed radial velocity of the hydrogen clouds determines only the distance to the galactic centre. So two possible values of the distance to the sun correspond to one value of the frequency. We have used as a criterion to separate the contributions from the two regions the latitude distribution of the radiation.

Two years ago I reported at the Jodrell Bank symposium on the initiation of the Agassiz Station project in Radio Astronomy. At that time the 24-ft. antenna was under construction and the electronic equipment for observation of the 21-cm. line of neutral hydrogen was being built by Harold I. Ewen, who is the co-director of the project. The first successful observations were obtained in the autumn of 1953 and reported early in 1954 (see Bok[1]). Since then two papers have been published giving the results of the first studies by Heeschen[2] and Lilley[3]; a report on the equipment and the basic programme was given by Bok and Ewen[4], Earlier this year, at the Princeton Meeting of the American Astronomical Society, Bok[5] drew heavily upon the Agassiz Station results in a paper entitled ‘Gas and Dust in Interstellar Clouds’, in which an attempt was made to blend the results of radio and optical research. We note also at this point a joint paper by Heeschen and Lilley[6] in which attention was drawn to the important role of Gould's Belt in the distribution of neutral hydrogen in the vicinity of the sun.

Observations of the 21-cm. hydrogen profiles have been taken between galactic longitudes l = 60° and l = 135°, with the 24-ft. radio telescope at the George R. Agassiz Station. The beam-width of the antenna is approximately 1°·7 between the half-power points. The electronic equipment used was a d.c. comparison radiometer, which has a signal channel with a frequency band-width of 15 kc./s. between the half-power points. At galactic latitude b = 0° the observations are generally spaced 2°·5 apart, although a few gaps of 5° exist. At b = + 15° and b = ሢ 15° the centres are 5° apart. A strip in latitude at l = 100°, and other centres at various latitudes (mainly at l = 75° and l = 87°·5) have also been taken, but little reference will be made to this material in the present paper.

An analysis of the profiles of the 21-cm. radiation from neutral hydrogen promises to be of great importance for a study of the internal motions of specific regions of the Galaxy. The two factors which influence the shape of the profiles are the velocity distribution and the density distribution of the neutral hydrogen atoms in the line of sight. The velocity distribution is essentially determined by three factors (1) galactic rotation, (2) the random motions of the gases, and (3) local peculiar motions as, for example, expansion. In the plane of the Galaxy in any specific direction the isolation of a region of particular interest is made difficult because of the superposition of the radiation along the entire line of sight. Hence regions at intermediate galactic latitudes are more suitable for study of internal motions than regions on or near the galactic equator. Also, for the study of peculiar motions, regions with small galactic rotation terms have the distinct advantage that any prevailing preferential motion will be clearly indicated by the profiles. The Orion region satisfies most of the above requirements. The galactic latitude of the section under consideration falls between − 10° and − 25°, and at the mean galactic longitude of 170° the galactic rotation term in radial velocity amounts to 7 km./sec. at a distance of 500 parsecs. Moreover the Orion region contains many features of considerable interest like the Orion Nebula, the Orion Association, the great arc of ionized hydrogen and many smaller H II regions. The great arc of Barnard (1895) [1] forms part of an almost elliptical ring of emission nebulosity with dimensions 14 × 12°. At the distance of 500 parsecs for the Orion Association these dimensions are of the order of 120 × 105 parsecs. It is of interest to note that the major axis of this ellipse is parallel to the galactic equator. This ellipticity could presumably be caused by galactic rotation, by a galactic magnetic field with lines of force along the spiral arms, or by the rotation of the whole mass itself. Further investigation is necessary to decide which of the above effects is most important.

The detailed relationship between optical interstellar absorption lines and 21-cm. observations is investigated in this paper.

Dr Guido Münch, of the Mount Wilson and Palomar Observatories, provided the list of six intermediate-latitude stars shown in Table 1. The spectra of these stars all show complex absorption lines due to interstellar Ca 11. The 21-cm. line is measurable in four of the six regions, although the peak intensity is low in each case. It is noteworthy that for the first two stars on the list the radio velocity agrees closely with the velocity of an intense optical component. In view of the great difference in angular resolution, the failure to find correspondence in every case is not surprising.

The meridianal surveys [1] at 21-cm. of interstellar hydrogen gas have been continued. Surveys at galactic longitudes of 50° [1], 60° [2], 80° [2], 90° [1], 110° and 200° have been completed while partial surveys have been made at longitudes of 20°, 205°, and 210°.

In radio astronomy it is becoming increasingly important to know the distance of the radio sources. An identification with astronomical objects observed optically is then more readily obtained and this in turn may allow further investigation of the mechanism of radio emission. A measurement of the distance of sources will also resolve the problem of their distribution in space, showing which are galactic and which are extra-galactic. Furthermore the surface area and absolute luminosity can be estimated from a knowledge of the distance and angular size of a source.

In the discussion which follows, there are presented three areas of the Naval Research Laboratory's study of absorption effects in the spectra of discrete sources produced by interstellar hydrogen. The first topic presented is the theory of the two mechanisms which are operative in producing the observed absorption effects. The second area of the discussion presents the observations and their interpretations for sources which lie in directions where the effects of galactic rotation are an important aid in the interpretation of the observed absorption. When galactic rotation effects are small, special techniques must be developed to supplement the absorption effect in order to determine distances to the discrete source. An example of the latter technique is contained in the third topic presented here, in a study of the source Sagittarius A. All the data discussed here were obtained with the Naval Research Laboratory's 21-cm. radiometer in conjunction with the 50-ft. antenna. A signal band width of 5 kc. (1 km./sec.) was employed throughout.

Reference to Fig. 1, p. 20 of this symposium will show that a 1° beam directed toward the galactic centre receives a range of velocities that is not negligible. This is due to galactic rotation, which causes material in one edge of the beam to approach and in the other edge to recede.

As was shown by I. S. Shklovsky, a radio spectral line from interstellar deuterium on λD = 91·6 cm. (fD = 327·38424 Mc./s.) may be expected [1]. According to Shklovsky the difference between the effective temperature Teff in this line and outside it must be of the order of 1/500 Teff in the direction of the galactic centre, if the concentration of deuterium equals 10−3 that of hydrogen. In this case, contrary to the emission by hydrogen, an absorption line is expected. As Teff equals about 300° the depth should be of the order of 0·6°.

A review of microwave and radio-frequency spectral lines which might possibly be detected by the techniques of radio astronomy is attempted here. Brief discussions of this type have already been given by several authors [1, 2, 3, 4]. However, the present treatment is somewhat more complete than previously published material, and has the advantage of more recent information about certain transition frequencies. It includes a general discussion of types of spectra which might be found, expected intensities, and some characteristics and known frequencies of the lines which may be of interest in radio astronomy.

Loose agreement of a radio position of low accuracy with that of some object listed in the NGC is not sufficient to provide the identification of a radio source. Even satisfactory coincidence of a precise position with that of an astronomical object requires supporting evidence. Agreement of the size of the source with that of the visible object, at least in order of magnitude, is an important argument in favour of an identification; exact agreement of sizes can be expected only where radio and optical emission are physically connected. The radio spectrum, the optical spectrum, and the physical characteristics of the visual object also have to be taken into account. Observations of the radio spectrum should be particularly useful to support the identification of sources with H 11 regions which can be recognized from their thermal emission even if they are obscured and optically inaccessible. If all data are available, satisfactory agreement exists between optical and radio observations. The best example of this kind at the moment is perhaps NGC 2237, the Rosette nebula, reported as a source by Ko and Krauss (1955) [1] and also observed by Mills, Little and Sheridan (1956 [11]; see also paper 18).

The study of continuous spectrum cosmic radio waves in the Radiophysics Laboratory is based mainly on two large ‘Mills Crosses’: one on 85 Mc./s. which has been in use for a year, and another on 20 Mc./s. which is under construction. The principle of this instrument has been described by Mills and Little (1953) [1]. It utilizes two long thin arrays arranged in the form of a cross and gives an effective ‘pencil-beam’ response which records both the background and discrete sources simultaneously. Essential data relating to these Crosses are given in Table 1.

Research in radio astronomy at the Ohio State University covers a wide range of astronomical and engineering phases. In this paper the principal topics currently under investigation are briefly discussed. Reference is made to more detailed papers or reports published within recent months or now in the press.