Expanding Horizons – The Milky Way and Beyond

Orchiston, Wayne; Robertson, Peter; Sullivan III, Woodruff T.

doi:10.1007/978-3-319-91843-3_4

Wayne Orchiston^16,17,
Peter Robertson¹⁸ &
Woodruff T. Sullivan III¹⁹

Part of the book series: Historical & Cultural Astronomy ((HCA))

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Abstract

Non-solar research at the Radiophysics Laboratory (RP) was launched in September 1946 when Joe Pawsey (1908–1962) tried unsuccessfully to observe the enigmatic ‘radio star’ in Cygnus that the British team of Stanley Hey (1909–2000, Fig. 4.1), John Parsons and James Phillips (1946) had announced in the 17 August issue of Nature. Two months later, John Bolton (1922–1993; Fig. 4.2) and research assistant Bruce Slee (1924–2016) were at Dover Heights trying to observe the Sun at 60 MHz. When it insisted on remaining inactive they decided to use their 2-Yagi antenna in sea interferometer mode to search for radio emission from other types of objects. Neither had a background in astronomy, and their astronomical knowledge and resources were virtually non-existent. Bolton (1982: 349) later described how they used the Russell, Duggan and Stewart book Astronomy to “… hazard guesses as to which types of objects might emit copious amounts of radio emission …” and Norton’s Star Atlas “… to find the position of the brightest candidate in each class.”

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4.1 Discovery of the First Discrete Radio Sources

Non-solar research at the Radiophysics Laboratory (RP) was launched in September 1946 when Joe Pawsey (1908–1962) tried unsuccessfully to observe the enigmatic ‘radio star’ in Cygnus that the British team of Stanley Hey (1909–2000, Fig. 4.1), John Parsons and James Phillips had announced in the 17 August 1946 issue of Nature (Hey et al., 1946). Two months later, John Bolton (1922–1993; Fig. 4.2) and research assistant Bruce Slee (1924–2016) were at Dover Heights trying to observe the Sun at 60 MHz. When it insisted on remaining inactive they decided to use their 2-Yagi antenna in sea interferometer mode to search for radio emission from other types of objects. Neither had a background in astronomy, and their astronomical knowledge and resources were virtually non-existent. Bolton (1982: 349) later described how they used the Russell, Duggan and Stewart book Astronomy to “… hazard guesses as to which types of objects might emit copious amounts of radio emission …” and Norton’s Star Atlas “… to find the position of the brightest candidate in each class.”

Their search was both unsuccessful and unauthorised, and on this latter count attracted the ire of their boss, Joe Pawsey (Fig. 4.3): “Our efforts … were cut short by an unheralded visit from Pawsey, who noted that the aerials were not looking at the Sun. Suffice it to say that he was not amused and we were both ordered back to the Lab.” (Bolton, 1982: 349–350).

By early 1947 Bolton had done ample penance and was allowed to return to Dover Heights, but this time with Pawsey’s blessing he searched for the Cygnus source and other radio stars whenever the Sun was inactive. By June it was, and Bolton teamed with receiver specialist Gordon Stanley (1921–2001) to search for the Cygnus source with the 100 MHz sea interferometer. They easily detected the source (Fig. 4.4), and saw ample evidence of the anomalous intensity variations reported by Stanley Hey and his group. Meanwhile, the interference fringes showed that this radio star was less than 8 arcmin in diameter, the first evidence for the very small angular size of the source. Subsequently, daily rising patterns with the antenna pointed in various north–east directions yielded a very approximate position. Over the next two to three months the Cygnus source was also detected with the 60, 85 and 200 MHz Yagis and a rough radio spectrum was plotted (subsequently found to be inaccurate). Their exciting results were reported in issues of both Nature (Bolton and Stanley, 1948b) and the Australian Journal of Scientific Research (Bolton and Stanley, 1948a; Fig. 4.5). In 1950 Jack Piddington (1910–1997; Fig. 3.33) and Harry Minnett (1917–2003; Fig. 3.34) extended the spectrum to 1210 MHz, and confirmed the non-thermal nature of the emission.

In November 1947 Bruce Slee joined Bolton and Stanley, and the three young researchers used aerials at Dover Heights and at Long Reef and West Head, respectively 15 km and 35 km to the north (see Fig. 1.14), and measured intensity variations from the Cygnus source. They found there was a high correlation at the three sites, showing that the variations were either an intrinsic feature of the source itself or else were caused by an ionospheric or interplanetary diffraction pattern with electron density turbulence scale much larger than the 15 km spacing between the two nearest observing sites.

The pressure was now on to search for more radio stars, which led to a survey with the 100 MHz sea interferometer. Starting in November 1947, the observational work involved several nights on each of about 15 azimuth settings separated by about 10 degrees. If a weak interference pattern was suspected, the azimuthal position was checked several times before the existence of a source was accepted (the intensities of these new sources were about five times weaker than that of Cygnus). A second new radio star was found on 6 November in the Taurus constellation, and it was followed shortly after by two others in the Centaurus and Virgo constellations. See Orchiston and Slee (2006) and Robertson et al. (2010, 2014) for detailed accounts of the discovery of the Taurus and Centaurus sources respectively.

By the end of 1947 a fifth and sixth source had been discovered. To keep track of this growing list, Bolton decided to name each source after the constellation in which it was found, followed by a letter A, B, …to indicate that it was the strongest, second strongest source, etc. in that particular constellation. This naming convention was later adopted by radio astronomers around the world and it is still partly in use today.

These detections were quite some achievement given the primitive nature of the equipment, which

… was very cranky … you got interference patterns one day and wouldn’t get them the next. Equipment would fail … The sea interferometer had a lot of nasty habits, like you could get interference wiped out by refraction problems and get sources rising ten minutes of time late and all this sort of crazy stuff … (Stanley, 1974).

The positions obtained for these new sources were too inaccurate to attempt optical identifications, although Bolton, Slee and Stanley reasoned that they lay well beyond the Solar System and that they generated extremely-high levels of radio emission.

The Dover Heights group saw it as their first priority to obtain more accurate positions so that they could seek optical correlates for these radio stars, and what they required were much higher cliffs that would also allow observations of the sources when both rising and setting. They eventually selected two sites in the North Island of New Zealand, with coastal cliffs about 300 m above sea level, three times higher than the Dover Heights site. Stanley was responsible for designing the mobile laboratory on which four 100 MHz Yagis were mounted in sea-interferometer mode (Fig. 2.15). Between early June and early August in 1948, Bolton and Stanley carried out observations at Pakiri Hill and then Piha near Auckland (Fig. 4.6), often under appalling weather conditions, but the observations produced excellent interference fringes (e.g. see Fig. 4.7) which yielded much better positions for Cygnus A, Taurus A, Virgo A and Centaurus A. These were precise enough to allow tentative optical identifications for the last three sources with bright optical objects (see Table 4.1). Taurus A was associated with a Galactic supernova remnant known as the Crab Nebula, but the other two optical objects (NGC 4486 and NGC 5128 respectively) were peculiar (see Fig. 4.8), and as such it was not possible to determine whether they were Galactic or extragalactic in nature. To their disappointment, the Dover Heights group could not find any obvious optical object within the error box of the strongest radio star, Cygnus A. Despite this shortcoming, the RP Chief Taffy Bowen (1911–1991; 1973) suggested that the Bolton, Stanley and Slee (1949) paper in Nature was one of the most important ones published by RP in the early years of radio astronomy (e.g. see Fig. 4.9).

Table 4.1 Three radio sources and their possible associated visible objects (adapted from Bolton, Stanley and Slee, 1949: 101)

Full size table

Actually, with the benefit of hindsight, we regard the Nature paper as the most important one published by RP in the early years of radio astronomy. It marked the birth of a new and dynamic branch of astronomy – extragalactic radio astronomy – which led to many of the exciting discoveries in astronomy during the second half of the twentieth century. For detailed studies of the discovery of the first discrete sources at RP see Robertson et al. (2014), Orchiston (2016c) and Robertson (2016, 2017).

One other important outcome of the New Zealand field trip was information about the enigmatic intensity fluctuations exhibited by Cygnus A (Fig. 4.4). Simultaneous observations of this source and the active Sun made by Slee at Dover Heights and by Bolton and Stanley in New Zealand showed conclusively that the Cygnus A variations were uncorrelated, while the solar bursts were in good agreement. Bolton, Slee and Stanley were now certain that the Cygnus A fluctuations were not intrinsic to the source, but were probably scintillations caused by diffraction in the intervening medium, with the scale size of electron density turbulence less than 2000 km. Unfortunately, Stanley and Slee (1950) did not publish this important result until late 1950. In the meantime, prompted by a letter from Pawsey (1949) to Martin Ryle, the Cambridge and Jodrell Bank groups carried out joint observations on Cygnus and, with a baseline of over 200 km, were easily able to establish an ionospheric origin for the fluctuations, with scale sizes from 5 to 10 km. The results were quickly published in Nature (Smith, 1950; Little and Lovell, 1950), but made no mention of the Radiophysics work (Sullivan, 2009: 324–27). When Lovell later learnt of the oversight he publically apologised to the Australians, but the Cambridge group remained silent. This marked the beginning of an escalating tension between the Sydney and Cambridge groups (see below).

Following the New Zealand field trip, the search was on in earnest for more radio stars. During 1949 Bolton, Stanley, Slee and Kevin Westfold (1921–2001; Fig. 1.27) used the equatorially-mounted 9-Yagi array at Dover Heights (Fig. 4.10) to survey the sky at 100 MHz for new radio sources and to investigate the properties of known ones. This work produced 14 additional radio stars, including Hydra A, Hercules A, Fornax A and Pictor A. Stanley and Slee also used a number of 2-Yagi antennas to measure the intensities of the four strongest radio stars at a number of frequencies over the range 40–160 MHz, and published much more accurate source spectra (Fig. 4.11). These strongly suggested that the radio emission was non-thermal in origin. Bruce Slee (1978) later recalled that this was an exciting time to be involved in radio astronomy, for “… you could expect to find a new source almost every night.” (see Bioboxes 4.1 and 4.2 for Slee and Stanley).

Biobox 4.1: Bruce Slee

Owen Bruce Slee (Fig. 4.12) was born in Adelaide on 10 August 1924 (Orchiston, 2005; Orchiston, Sim and Robertson, 2016). He joined the Radiophysics Lab in 1946 after serving at radar stations during WWII and independently detecting solar radio emission at 200 MHz. Studying evenings, he completed a BSc (First Class Honours) at the University of New South Wales in 1959, and subsequently was awarded a DSc by the same university in 1971.

After serving with the Dover Heights team that elucidated the true nature of ‘radio stars’ (Slee, 1994), he gained further international recognition during the 1950s through the Mills–Slee–Hill catalogue of radio sources at Fleurs, and the controversy this generated vis-à-vis the Cambridge 2C survey. Bruce then went on to pioneer the detection of radio emission from various types of active stars. Over the years he also made important contributions to pulsar astronomy and to the study of radio galaxies and clusters of galaxies. After retiring from the Australia Telescope National Facility in 1989 (as a Principal Research Scientist), he was made an ATNF Honorary Fellow and continued an active research program using the Australia Telescope Compact Array. He published extensively on a wide range of astronomical topics and, in recent years, also wrote a series of co-authored papers with Wayne Orchiston on the early history of Australian radio astronomy.

Bruce Slee “… is a pioneering Australian radio astronomer, and it is a testimony to his dedication and his passion for astronomy that after more than half a century he continues an active research program. Since joining the CSIRO’s Division of Radiophysics in 1946 he has made an important, long-term, wide-ranging contribution to astronomy and to Australian science …” (Orchiston, 2004: 69). Bruce Slee died on the south coast of New South Wales on 18 August 2016. In 2016 the International Astronomical Union named minor planet 9391 after him, and in 2017 he was posthumously awarded a Member of the Order of Australia (AM) for his distinguished service to radio astronomy.

Biobox 4.2: Gordon Stanley

Gordon James Stanley (Fig. 4.13) was born in Cambridge, New Zealand, on 1 July 1921. After his family moved to Sydney, Gordon first worked for a manufacturer of electrical appliances and developed skills that would be essential to his future career. In 1943 he joined the Radiophysics Lab and developed expertise in designing and building receivers and antennas. In 1945 he received an engineering degree from Sydney Technical College and was promoted to Technical Officer at Radiophysics.

Early in 1947 Gordon joined John Bolton at the Dover Heights field station and they were able to show that intense radio emission from the Cygnus constellation came from a discrete, point-like source. Bruce Slee joined them late in 1947, and as a team they obtained accurate positions for three more of these discrete radio sources, Taurus A, Centaurus A and Virgo A, and this enabled them to identify the optical counterparts of these sources. This landmark achievement was published in a 1949 Nature paper (Bolton, Stanley and Slee 1949) and soon brought international recognition to the Dover Heights group and to Australian radio astronomy.

In 1955 Gordon and John Bolton were recruited by the California Institute of Technology (Caltech) in Pasadena to establish a radio astronomy group. They founded the Owens Valley Radio Observatory (OVRO) in northern California and Gordon designed the electronics for an interferometer consisting of two 27 m parabolic dishes (see Fig. 1.22). With Bolton’s return to Australia in 1960, Stanley was subsequently appointed Director of the OVRO. During the 1960s he consolidated Caltech’s position as one of the world’s leading centres for radio astronomy.

After retiring from Caltech in 1975 he returned to his primary love of radio engineering. In 1982 he formed Stanley Microwave Systems which built microwave instruments for atmospheric and oceanographic applications. He spent his final years in Carmel Valley with wife Helen, where he died on 17 December 2001. For a memoir see Kellermann, Orchiston and Slee (2005).

At the Potts Hill field station two other RP staff, Bernie Mills (1920–2011; Fig. 2.52) and Adin Thomas, were particularly interested in measuring accurate positions for the strongest radio stars, and they approached this challenge from another direction. Between May and December 1949 they carried out observations with the 97 MHz position interferometer (Fig. 2.39) developed earlier for solar work by Ross Treharne (1919–1982) and Alec Little (1925–1985) (Orchiston and Wendt, 2017). Mills and Thomas used the times of transit to establish the right ascensions of these sources and the periodicity of the interference patterns to determine their declinations. After applying various corrections they derived co-ordinates of R.A. 19h 57m 37 ± 6s and Dec. +40° 34 ± 3′ for Cygnus A, but were disappointed that within this error box “… there is no prominent or unusual celestial object with which the source can be identified” (Mills and Thomas, 1951: 170). However, they did note the presence of a faint extragalactic nebula just outside their error box (3′ away). They also found that the radio star was less than 3 arcmin in size, and that its characteristic fluctuations in intensity could be explained by terrestrial atmospheric effects. This was Mills’ first excursion into non-solar astronomy, and followed lengthy discussions with Pawsey about fruitful new research prospects (see Sullivan, 2009: 337–339).

Mills continued this investigation when he moved to the Badgery’s Creek field station, motivated by nagging concerns about the optical identifications suggested by Bolton, Stanley and Slee for Centaurus A and Virgo A: “… we always felt in the back of our minds that they may not be exactly right. This is one of the reasons I … built bigger antennas with the idea of getting more accurate positions with a larger number of sources” (Mills, 1976). Between February and December 1950 and for a short interval in 1951 he obtained further positional measurements for six prominent radio stars using three 101 MHz broadside arrays (Fig. 2.53). The results are listed in Table 4.2. The positions obtained for Taurus A, Centaurus A and Virgo A actually reinforced the optical identifications suggested by the Dover Heights group and confirmed the extragalactic nature of the last two sources, but it was the position obtained for Cygnus A that had greatest impact. Mills’ value was published in the Australian Journal of Scientific Research (Mills, 1952a) and it coincided with the position published in Nature in 1951 by Graham Smith (b. 1923). Moreover, within the error boxes of both sets of values was a faint extragalactic nebula (the same one that Mills and Thomas had noted just outside their earlier error box), and closer examination of this object by Walter Baade (1893–1960) and Rudolph Minkowski (1895–1976) with the 200-inch Palomar Telescope in California (Fig. 4.14) revealed what appeared to be two faint colliding galaxies (see Fig. 4.15) (Baade and Minkowski, 1954).

Table 4.2 Positions of early radio stars, based on observations at Badgery’s Creek (after Mills, 1952b: 461)

Full size table

Now only the Fornax A and Hydra A sources remained to be optically identified, although Mills suspected that NGC 1316 was connected with the former. Later he used the benefit of hindsight to comment on this:

Fornax A is a double source and although the centroid is nicely on the galaxy, in fact the fine structure is more distributed on one side than the other so that the interferometer position is weighted towards the position of the fine structure. We now know what’s happening, but at that time it was not clear. (Mills, 1976).

By 1952 the extragalactic nature of Cygnus A was thus established, and the Virgo A and Centaurus A sources were also confirmed to be extragalactic nebulae. Only Taurus A was associated with an object inside our Galaxy. These ‘radio stars’ seemed not at all to be stars, but mostly ‘radio galaxies’, distant objects that generated extraordinary amounts of radio energy. Stanley Hey in England had originally proposed that the Cygnus source was a strange radio-emitting star in our Galaxy, and most other radio researchers agreed for many years, but this idea was now put to rest. It would take another decade before radio telescopes evolved to the point where they could indeed detect radio emission from stars other than the Sun – and, ironically, one of the leaders in this new field of radio astronomy was none other than ‘radio star’ pioneer Bruce Slee.

This change in understanding saw a commensurate shift in terminology, and ‘radio stars’ became ‘discrete sources’, a generic term that had no specific Galactic or extragalactic connotation. The research focus shifted to establishing the celestial distribution of discrete sources at various frequencies, and in Australia this fitted perfectly with the strategy of maintaining small, largely autonomous groups of scientists at a number of different field stations. Several research teams directed their attention to surveys of these discrete sources.

For example, between June 1950 and June 1951, Alex Shain (1922–1960) and research assistant Charlie Higgins conducted an 18.3 MHz source survey of the sky mainly between –4° and –60° declination using a 30-element array at the Hornsby Valley field station. Altogether, 37 different discrete sources were detected, some of which exhibited intensity fluctuations. Only approximate positions were obtained for the weaker sources, but when all 37 were plotted (Fig. 4.16), they exhibited a broad distribution. For further details of this research see Orchiston et al. (2015).

At this time Bernie Mills was based at Badgery’s Creek, and in 1950 he carried out a 101 MHz survey of the sky between declination +50° and –90° with a view to studying the Galactic distribution of discrete sources. He detected 77 sources (Fig. 4.17), and concluded that there were two types: (1) stronger sources, which were predominantly Galactic in nature because they were clustered close to the Galactic Plane, and (2) weaker sources, which were more randomly distributed over the sky. These latter sources might have been either extragalactic sources or weak Galactic sources relatively close to the Sun. This was a very different general picture to the one obtained by the Cambridge radio astronomers, as Mills (1976) was later to reflect:

[The Cambridge radio astronomers] thought in terms of point sources uniformly scattered over the sky, radio stars in fact. And in the south we had measured all these large angular sizes and it was obviously much more complex than this … They had missed the strong sources near the plane [of the Galaxy] because they were using wide-spaced interferometers … This was the basic cause of our arguments at that time with Cambridge …

At Dover Heights, John Bolton, Gordon Stanley and Bruce Slee carried out a comparable survey at the similar frequency of 100 MHz in 1951 using the new 12-Yagi array shown in Fig. 2.21 (Bolton et al., 1954a; see also Bolton et al., 1954b). According to Bolton (1978), this survey involved about two years’ observing, largely because of problems generated by the ionosphere and thunderstorms. Sometimes “… there was only one night a week that you got good observations.” Nonetheless, this work resulted in the detection of 104 sources, with flux densities down to 40 Jy. This was the most comprehensive list of radio sources available at the time, and showed that discrete sources were far from rare: increasing sensitivity seemed to bring increasing numbers of them (see Table 4.3 for a summary of the source surveys carried out over the period 1950–1954). The celestial distribution of the 104 sources is shown in Fig. 4.18, where the predominance of stronger sources along the plane of the Galaxy and of weaker sources in the southern polar cap is apparent. Meanwhile, new optical identifications suggested by Bolton, Stanley and Slee only served to reinforce the view that most discrete sources were extragalactic in origin and associated with radio galaxies.

Table 4.3 Surveys of discrete radio sources 1950–54 (adapted from Bolton, Stanley and Slee, 1954a: 111)

Full size table

It is interesting that at this time there was little contact between the Dover Heights and Badgery’s Creek groups. According to Mills (1976), this was because

We each felt rather strongly our own technique was the best. Although we saw each other sometimes, Bolton lived out at Dover Heights and didn’t come into the Lab very often and I spent most of my time out at Badgery’s Creek. So we didn’t have very much contact actually. And there were quite a few arguments about interpretation of the results and things like that.

The first attempt to look at discrete sources at much higher frequencies occurred in 1950 when Jack Piddington and Harry Minnett carried out a survey at 1210 and 3000 MHz from Potts Hill using the ex-Georges Heights radar antenna and two small parabolic antennas (see Figs 2.36 and 2.38) (Piddington and Minnett, 1951). They detected many of the stronger sources and, as a result of their Crab Nebula observations, Piddington later went on to theoretically investigate its associated magnetic field, coming up with a rapidly-rotating star. “That was the first, as far as I know, suggestion of a – what are they called now – rators?” (Piddington, 1978). What he was actually referring to was then known as a ‘spinar’. Only two decades later would the neutron star (pulsar) at the centre of the Crab Nebula become known.

In 1953 Slee, Stanley and a new recruit to the Dover Heights group, Dick McGee (1921–2012), were able to learn something about discrete source emission at an intermediate frequency when they carried out a survey at 400 MHz using the 24.4 m ‘hole-in-the-ground’ antenna at Dover Heights (see Section 4.3 below). This small-scale survey concentrated on the Milky Way between declinations –17° and –49°, and the method of observation was described by McGee, Slee and Stanley (1955: 353):

The Milky Way was observed on one fixed declination per day, changes in Right Ascension occurring as the rotation of the Earth swept the aerial beam across the sky. In a preliminary survey observations were made at intervals of 1° in declination. Later, in order to cover the more interesting regions in greater detail, intervals of ½° were used.

This survey produced a list of just 14 new sources, and with the benefit of hindsight and improved resolution we now know that about half of these were the result of confusion – instances where several adjacent sources were unresolved by the 2° beam of the antenna.

Further details of the ‘hole-in-the-ground’ research are presented in Orchiston and Slee (2002). For reviews of the various ‘source surveys’ conducted at Dover Heights and Potts Hill field stations see Orchiston and Robertson (2017) and Wendt et al. (2011b), respectively. For a detailed overview of the history of worldwide research on discrete sources up until 1953 see Sullivan (2009).

4.2 From Dover Heights to Fleurs

A major advance in the Australian study of discrete sources took place once the 85.5 MHz Mills Cross at Fleurs (Fig. 2.59) became operational in 1954. This new radio telescope was used for three different projects. Soon after its inauguration, Bernie Mills, Alec Little and Kevin Sheridan (1918–2010; Fig. 3.23) searched for radio emission from specific types of celestial objects, including known novae, supernovae, and planetary nebulae. Unfortunately, they were spectacularly unsuccessful, the only object to reveal itself as a radio source being the Kepler supernova remnant of 1604. Nonetheless, Mills et al. (1956b: 84) boldly suggested that “… galactic radio emission near the plane of the Milky Way could be largely the integrated emission of supernova remnants …”.

Mills and his colleagues had more success when they tried to observe 14 emission nebulae in our Galaxy, the Magellanic Clouds, and 11 bright southern galaxies with the new Cross. Six of the nebulae were detected in emission and one in absorption, and crude isophote maps were prepared for two of these (Fig. 4.19). Both Magellanic Clouds were detected (Fig. 4.20), as were eight of the 11 galaxies. Mills (1955: 369) found that these observations and others made with the Mills Cross in the general vicinity of the Galactic Centre

… tend to support the ideas of [Russian theorist] Shklovsky [1916–1985] in which the galactic emission is considered to be distributed in two subsystems, one showing a discoidal distribution highly concentrated towards the galactic plane, and the other a very dispersed and approximately spherical distribution concentric with the galactic centre.

As we shall see, Mills would return to this ‘disk–corona’ dichotomy in later publications.

Sheridan (1958) also used the Mills Cross to study some of the strongest known radio sources, and was able to produce isophote maps for Centaurus A, Fornax A and Puppis A (Fig. 4.21). The Centaurus A and Fornax A results nicely complemented similar 19.7 MHz plots published at the same time by Shain (1958), based on observations made with the new Shain Cross at Fleurs (Fig. 2.63), and one of these is included here in Fig. 4.22 (left). For comparison Fig. 4.22 (centre) shows the 1400 MHz Centaurus A isophote plot that was produced at this time by Jim Hindman (1919–1999) and American visitor Campbell Wade (b. 1930) using the 11 m parabolic antenna at Potts Hill (Fig. 2.43).

One of the most unexpected findings at 85.5 MHz was that Centaurus A extends far beyond the boundaries of its associated optical galaxy. Furthermore, when Wade subtracted the intense localised central source he was left with two broad maxima, as shown in Fig. 4.22 (right). Upon reviewing the evidence at 1400 MHz, Hindman and Wade (1959: 268) came to a similar conclusion: “The source consists of two components: one is a point source evidently associated with the peculiar galaxy NGC 5128; the other is a very large extended source with no known optical counterpart.”

The above-mentioned Mills Cross projects were interesting, but they were simply forerunners to the main work of the Cross which was to produce a detailed survey of the sky at 85.5 MHz. The observations for this were carried out by Bernie Mills, Eric Hill (1927–2016) and Bruce Slee between 1954 and 1957, and in the process they recorded about 2300 sources of discrete radio emission, publishing their results in a series of research papers in the Australian Journal of Physics (e.g. Mills and Slee, 1957). Although a number of the sources in this MSH Catalogue were associated with Galactic objects, the majority related to extragalactic nebulae. Edge and Mulkay (1976) and co-author Sullivan (1990) have described how this had profound cosmological implications in terms of the competing ‘Big Bang’ and ‘Steady State’ theories which were prevalent at the time.

The MSH survey strongly disagreed with a parallel survey (called 2C) by Martin Ryle’s (1918–1984; Fig. 1.39) group at Cambridge University. In the overlap region between the northern and southern surveys there was hardly any agreement between the very existence of individual radio sources (Fig. 4.23). Statistical studies of the histograms of source intensities also disagreed (Fig. 4.24).

According to Mills (1976), this issue had its origin when the prominent English astronomer Fred Hoyle (1915–2001) wrote a letter to the RP radio astronomers in which he mentioned “… a very great excess of faint sources which they had obtained at Cambridge, … and asking what did we get.” At the time, Mills et al. had only preliminary results – which could not possibly agree, as he responded to Hoyle. When they examined the 2C evidence, Mills and Slee noticed that the Cambridge radio astronomers “… were not listing radio sources as such, because one could take an area where there were some quite strong sources in the Cambridge catalogue, look at our records, and see that there was absolutely nothing there.”

Mills and Slee immediately came up with resolution effects as the explanation, but when Mills tried to raise this with Ryle his letters went unanswered. Later he was to remark: “We were a bit fed up with the Cambridge attitude at this time, I might say … They just ignored us. So we went ahead and did what we felt had to be done.” (Mills, 1976). This was to publish a paper reporting the MSH–2C discrepancies and explain them largely in terms of shortcomings associated with the Cambridge interferometer.

A heated controversy erupted, and others who examined the evidence drew their own conclusions. By this time, John Bolton (1955a) had taken up a position at Caltech, and although he thought Ryle’s work looked convincing on the surface he was quick to note that “… interferometers can be very misleading without some cross-check.” Later in the year he had a chance to look over the Cambridge equipment and results, and “… I was horrified at their records and the amount of information that they had tried to dig out of the confusion.” (Bolton, 1955b). Joe Pawsey and RP solar astronomer Paul Wild (1923–2008) were at a Jodrell Bank symposium in 1955 when Martin Ryle presented a paper on his source counts data, and Wild (1978) later remembered Pawsey

… doing a magnificent rebuttal in a way, very modestly, on the basis of Mills’ preliminary results. And Pawsey’s performance reminded me of Marc Antony’s speech, “Brutus is an honourable man”, in reference to Martin Ryle. But he absolutely won the day, and I’ve never seen better form from Pawsey, who was normally such a quiet, unassuming, non-confrontational individual.

The Manchester radio astronomers also had problems with the Cambridge results. In January 1957, Robert Hanbury Brown (1916–2002; Fig. 4.25) – ever the diplomat – informed Pawsey that “Our present results suggest that there is a great deal wrong with the Cambridge survey, but please do not quote me.” (Hanbury Brown, 1957; our italics).

Yet not everyone saw things this way. David Edge (1932–2003) who joined the Cambridge group after the completion of the 2C survey, later retrospectively commented: “The fact is that both surveys were severely limited – one by confusion [the 2C], the other by sensitivity [Mills Cross] … the two surveys were simply incompatible …” (Edge, 1975). Mills later commented on the many extended sources in his survey (but not in Ryle’s):

… we know now [1976] that a lot of these extended ones were [Galactic] background irregularities. We thought at the time they might be, but we included all these in the catalogue because, again, we didn’t know what we had to look for at that time. And any concentration of emission, of any sort, appeared to be worth cataloguing. (Mills, 1976).

Despite these MSH shortcomings, in the end it took some years before the more serious problems associated with the Cambridge survey were fully recognised and the Australian results were largely accepted. In hindsight, it is now clear that the instrumentation used in both surveys came with its own inherent problems.

The Mills Cross survey also came at a time when astronomers were busy trying to explain discrete sources. Back in 1953, when Mills visited Minkowski and Baade at Caltech,

It was all colliding galaxies … [and] there were a lot of discussions about probability of collisions, and how many there should be. There were some worries because there didn’t seem to be enough collisions [to account] for the radio sources which were observed. There was very little physics in that part of it. It was purely looking for abnormalities in galaxies which might be identified with radio sources. But no one had a clue about the physical connection. (Mills, 1976).

Confusing the issue was the fact that normal galaxies – like our own and the Great Nebula in Andromeda – were also (weaker) sources of radio emission, so not all galaxies that generated radio waves were ‘abnormal’. Furthermore, it was obvious to Mills and colleagues that their extended sources could be either Galactic or extragalactic, and might be due to blending of sources: “You see, at that time we knew about radio galaxies, we knew about clusters of galaxies, so we thought … it was highly probable that some of these extended sources did, in fact, represent blends of radio galaxies.” (Mills, 1976).

And what about the prodigious energies involved? What were the associated emission mechanisms? By 1954 Mills had read papers by the Swedish physicist Hannes Alfvén (1908–1995), and the Swedish/Norwegian astrophysicist Nikolai Herlofson (1916–2001), and suspected that synchrotron radiation was involved, but it was only when he was introduced to the papers of the Soviet astrophysicist Iosif Shklovsky – now regarded as classics – that everything suddenly fell into place. Much later he admitted that in the mid-1950s “... my outlook was quite close to the Russian [one]. I was very impressed with the Shklovsky work … I think I was just generally thinking along similar lines.” (Mills, 1976).

Concurrently with the MSH survey, RP’s Alan Carter carried out an interferometric study of the sizes of selected Mills Cross sources using broadside arrays at Fleurs and Badgery’s Creek (Fig. 2.53). Carter (1956) reported that

From these observations we have about 50 reasonably reliable sizes, and about another 25 less reliable sizes. Many of these sources are about a minute or two of arc in diameter … but we do not think these sources are a representative sample of all sources down to our sensitivity level.

This inspired further studies, and in October 1956 Mills prepared a document titled “Proposed Source Size Measurements at Fleurs”, in which he described the need for a radio-link interferometer involving part or all of the Mills Cross and a remote array at a more distant site, Wallacia, in order to measure the sizes of yet smaller (in angular size) MSH sources. He elaborated:

The bread and butter aim of these measurements is to aid the identification of sources. Without angular size data it is often impossible to decide between several nebulae which may be within the limits of error in the position of a radio source. The provision of approximate angular sizes for the 1000 strongest sources could easily make the difference between 50 new identifications and 300. This difference arises because, without angular size data, one can only be reasonably certain of an identification between a bright source and a bright nebula; with size data, one can hope for bright source–faint nebula and faint source–bright nebula identifications.

More interesting for a long term program is the possibility of recognising the very distant and powerful sources which are too remote for the associated galaxy to be detected optically … these must be very rare objects, but their recognition and the measurement of their properties can provide cosmological information which is unattainable by any other means …

Finally it is possible that the angular size interferometer proposed will itself be capable of making some significant advances in cosmology … (Mills, 1956a).

This project would provide for the continued use of the Mills Cross long after completion of the MSH source survey.

The first new investigation of source sizes was conducted by Bruce Goddard, Arthur Watkinson and Bernie Mills in 1958 and 1959, and used the E–W arm of the Mills Cross, a 91.4 m section of the S arm of the Cross, and an identical 50-dipole N–S array at Wallacia (Fig. 4.26) (Goddard et al., 1960). Drawing on a sample of about 1200 sources, mostly from the MSH survey, they were able to use source size in conjunction with other data to suggest identifications for about 10% of the sources. Mills elaborated:

A total of 46 possible identifications with galaxies are listed and 55 possible identifications with clusters of galaxies, the great majority of which are new. Most of these galaxies are double systems … [but] it seems probable that many galaxies of completely normal appearance are very strong radio emitters … In many cases, the emission from clusters appears to be associated with a single galaxy or pair of galaxies in the cluster … (Mills, 1960: 550).

Mills then moved to the University of Sydney and with more pressing demands on his time it was left to Bruce Slee and visiting Cambridge radio astronomer, Peter Scheuer (ca.1930–2001), to take the source size project further. In 1961 and 1962 they used the E–W arm of the Mills Cross in conjunction with ‘barley-sugar’ arrays (see Fig. 4.27) erected at four different remote sites to the north and south of Fleurs to investigate a large number of MSH sources (Scheuer et al., 1963). Scheuer was responsible for the final reduction of the observations and he took all the chart records with him when he returned to England, but unfortunately never found the time to properly analyse the observations. Consequently, few results from this investigation were published, and Scheuer later admitted that this was a source of considerable embarrassment to him.

While Goddard and his collaborators were engaged in the above-mentioned source size project, Richard Twiss (1920–2005), Alan Carter and Alec Little (see Biobox 4.3) used various Chris Cross aerials as a 1427 MHz interferometer in order to investigate the structure and polarisation of a number of bright southern radio sources. Among other findings, they were able to show that Centaurus A consists of two distinct components, “… each of about 2½′ size between half-intensity points and separated by 5′.” (Twiss et al., 1960: 156), and that Taurus A was roughly elliptical in shape.

Biobox 4.3: Alec Little

Alec George Little (Fig. 4.28) was born in Sydney on 2 February 1925, and left school at age 15 to join the Radiophysics Lab. He soon became a junior laboratory assistant and then made rapid progress through the technical staff ranks. Alec was “… a keen and gifted technician … and was generally regarded as a young man who was going places” (Mills, 1985: 113). After WWII he worked briefly in the valve laboratory before transferring to the radio astronomy group, where his career would flourish.

In collaboration with Ross Treharne and then with Ruby Payne-Scott, Little developed an innovative solar interferometer (Wendt and Orchiston, 2019), and he then worked with Bernie Mills on the Mills Crosses at Potts Hill and Fleurs. Studying part-time at Sydney Technical College, he completed Certificate and Diploma courses before obtaining a BSc (First Class Honours) from the University of New South Wales in 1955. He then spent two years at Stanford University as a Research Associate, in the process obtaining an MSc in radio astronomy. When Bernie Mills joined the University of Sydney in 1960, Alec followed him there, accepting a lectureship in the School of Physics. He and Mills worked on the Molonglo Cross (Fig. 5.5), and in 1968 Alec was appointed Director of the Molonglo Radio Observatory and in 1985 an Associate Professor. Subsequently he was responsible for overseeing the conversion of the Cross into the Molonglo Observatory Synthesis Telescope (see next chapter). During this period, Alec was also associated with the Fleurs Synthesis Telescope.

In the early 1980s he was appointed a consultant to the CSIRO Australia Telescope project. A former President and Vice-President of the Astronomical Society of Australia, he also served as the Australian representative on URSI Commission 5 (Radio Astronomy). Alec Little played a major role in the development of Australian radio astronomy and “His cheerful enthusiasm, forthright personality and lack of any pretensions endeared him to all his colleagues …” (ibid.). He died suddenly from a heart attack on 20 March 1985.

With the installation of a 60-ft dish at the eastern end of the Chris Cross at Fleurs, the RP radio astronomers had access to a powerful new instrument, the Fleurs Compound Interferometer (Fig. 2.66), with which to study southern radio sources at 1440 MHz, and two teams took up this challenge. Norman Labrum (1921–2011), T. Krishnan (b. 1933), Warren Payten and Eric Harting observed eight well-known Galactic and extragalactic sources, deriving angular sizes and accurate positions for each. Further to the result reported by the Twiss team, they found the eastern and western components of Centaurus A to have sizes of 2.7′ and 2.1′, separated by 5.1′ (Fig. 4.29).

For their part, Don Mathewson (b. 1929), John Healey and John Rome plotted emission along the Galactic Plane, noting the presence of a chain of conspicuous discrete sources (Mathewson et al., 1962a). Some of these were thermal sources and associated with gaseous nebulae previously identified by optical astronomers, and others were non-thermal sources linked to supernova remnants. Orchiston and Mathewson (2009) provide further details of research carried out at Fleurs with the 60-ft dish, and a summary of its research after its transfer to Parkes is contained in Orchiston (2012).

4.3 Radio Emission from the Galactic Centre

Between May and November 1949, Alex Shain (Fig. 2.28) carried out a survey of radio emission from a zone of sky centred on –34° declination using the highly-directional 9.15 MHz transit array at the Hornsby Valley field station (Fig. 2.30). This zone of sky included the Galactic Plane and the Galactic Centre, and the resulting crude isophote plot showed enhanced radiation along the Plane with the highest flux levels in the vicinity of the Galactic Centre. Shain and Charlie Higgins (1954) were able to improve on this rather unexciting diagram between June 1950 and June 1951 when they carried out a more detailed survey with an enlarged 18.3 MHz array. Their new isophote plot (Fig. 4.30) showed a strong source in the region of the Galactic Centre. However, in reporting this interesting result they cautioned that the true intensity of this source may be under-represented because of gas absorption at 18.3 MHz in the direction of the Galactic Centre. For further details see Orchiston et al. (2015).

By the time Shain and Higgins belatedly published their results in 1954, several colleagues had already drawn attention to the distinctive discrete source located at the centre of our Galaxy. The first to do so were Jack Piddington and Harry Minnett who conducted preliminary observations at 1210 MHz with much better angular resolution, using a 3 m paraboloid at Potts Hill in 1948 and later the much larger ex-Georges Heights radar antenna at both 1210 and 3000 MHz in 1950. In their report, published in the Australian Journal of Scientific Research in 1951, they remarked on a “… new, and remarkably powerful, discrete source” (Fig. 4.31) at R.A. 17h 44m and Dec. –30° close to the assumed centre of our Galaxy. Piddington and Minnett (1951: 467) referred to this as “The Discrete Source in Sagittarius–Scorpius”, given that its location was on the border of these southern constellations. By drawing on unpublished data provided by Shain (18.3 MHz) and Mills (100 MHz) they were able to plot its spectrum, which indicated “… an optically thin, thermally radiating gas …” (Piddington and Minnett, 1951: 469).

Biobox 4.4: Dick McGee

Richard Xavier McGee (Fig. 4.32) was born in Sydney on 31 December 1921 and died on 19 December 2012 (Sim, 2013). During WWII he served in the army and in the air force. He later studied at the University of Sydney as a student with the post-war Commonwealth Reconstruction Training Scheme, obtaining a BSc with First Class Honours in 1950. He then joined Radiophysics and, after working at the Dover Heights and Murraybank field stations, he became a stalwart of the group researching emission from hydrogen and interstellar molecules with the Parkes Radio Telescope.

In 1969 Dick was awarded a DSc by the University of Sydney in recognition of his published work. He was also interested in astronomy outreach and the history of Australian radio astronomy, and for a time served as RP’s honorary historian. Along with Rosslyn Haynes, David Malin, and RP colleague Raymond Haynes, he produced the classic text Explorers of the Southern Sky. A History of Australian Astronomy (Cambridge University Press, 1996). A founding member of the Astronomical Society of Australia, Dick served as Editor of the Society’s research journal from 1971 to 1988.

On Dick’s retirement in 1986 his colleague and former Chairman of CSIRO, Paul Wild, wrote: “I want to thank you for your enormous contribution to the Division. Not only for your research contributions – the discovery of the centre of the Galaxy, pioneering work on HI, and huge works in unraveling the mysteries of galactic astronomy – but also for many other contributions which helped the world, and Radiophysics, to go round and be more enjoyable places …” (Wild, 1986).

At Dover Heights, John Bolton, Kevin Westfold, Gordon Stanley and Bruce Slee independently detected this Galactic Centre source in late 1951 when they carried out observations at 160 MHz with the 21.9 m prototype ‘hole-in-the-ground’ antenna (Fig. 2.22). Subsequent observations in 1953 by Slee, Stanley and new team member, Dick McGee (Biobox 4.4), with the concreted 24.4 m antenna at 400 MHz (Fig. 2.24) produced an impressive contour plot and it was they who coined the name ‘Sagittarius A’ for this source in papers published in Nature (see Fig. 4.33) and in the Australian Journal of Physics (McGee et al., 1955). Before these papers were published, Joe Pawsey sent a copy of the isophote plot to Walter Baade at the Mt Wilson and Palomar Observatories for his evaluation. His reply is illuminating:

Frankly, I jumped out of my chair the moment I saw what it meant. I have not the slightest doubt that you finally got the nucleus of our galaxy!! … Altogether I concluded about two years ago – after a careful examination of my 48 inch Schmidt plates of the nuclear region of our galaxy and thorough checking of all suspicious objects at the 200 inch – that there was positively no chance whatsoever to detect the nucleus of our galaxy in the optical range and that we had to await what you radio people could do about it … It is very improbable that the coincidence between inferred and observed position of the nucleus is accidental. (Baade, 1954).

As it turned out, the position of Sagittarius A was particularly meaningful, and eventually was adopted by the International Astronomical Union to define the Galactic Centre and the anchor point for a new Galactic co-ordinate system. Sagittarius A is now widely known by its abbreviated name Sgr-A.

Further information on the discrete source at the centre of our Galaxy was forthcoming in 1955, when Bernie Mills, Bruce Slee and Eric Hill used the newly-constructed Mills Cross at Fleurs to survey the Galactic Plane at 85.5 MHz. The much smaller beam of this powerful radio telescope resolved Sagittarius A into two distinct components separated by about 2°, superimposed on a bright background that extended along the Galactic Plane for several degrees. In a paper published in Observatory, Mills (1956b) concluded that this may be due to absorption caused by an HII region situated in front of, or partially in front of, the Sagittarius A source. Mills (1976) later reflected:

This was obviously an intensely interesting result. It was one of the few occasions I think where I sat down and wrote a paper very rapidly as soon as we had the contours worked out. It was clear that this non-thermal emission was arising on either side of the thermal emission.

Evidence of absorption along the Galactic Plane at even lower frequencies emerged at about this time following the commissioning of the new 19.7 MHz Shain Cross at Fleurs. Initially, the main research program involved the observation of a strip of sky extending some 10° on either side of the Galactic equator, and of particular interest was the appearance of the Galactic Centre. A map of the region (Fig. 4.34) revealed that Sagittarius A was indeed seen in absorption.

In a useful review paper published in 1959, Mills brought together all the Australian observations of Sagittarius and concluded that although several physical models explain the derived data,

… the most plausible one, and probably the correct one, consists of an HII region embedded in a flattened spheroidal non-thermal source, both being located at the galactic nucleus. The dimensions of the HII region would then be of the order of 89 pc × 35 pc and its mass some 2.5 × 10⁵ times that of the Sun. (Mills, 1959b: 281).

The final study of the Galactic Centre region conducted by RP radio astronomers prior to their assault on this region with the Parkes Radio Telescope came in April–May 1961 when Mathewson, Healey and Rome used the newly-constructed Fleurs Compound Interferometer with its 50′ beam. Their observations not only confirmed the discovery by the Dutch–American radio astronomer Gart Westerhout (1927–2012) of a ring of ionised hydrogen in the inner reaches of the Galaxy, but also suggested that near the Galactic Centre this hydrogen was “… concentrated in an irregular spiral structure … [with] peaks in the thermal emission at l^II = 330°, 338°, 14°, 27°, where the line-of-sight is tangential to the arms.” (Mathewson et al., 1962b: 374).

For more on the discovery of the Galactic Centre see Morton (1985) and Robertson and Bland-Hawthorn (2014). It is interesting to note that Andrea Ghez (University of California) and Reinhard Genzel (Max Planck Institute) were awarded the Nobel Prize for Physics in 2020 for their discovery of a supermassive black hole at the Galactic Centre.

4.4 Surveys of the Background Radio Emission

The first map of background radio emission from the sky – as visible from Sydney – was published by John Bolton and Kevin Westfold in the Australian Journal of Scientific Research in 1950 (see Fig. 4.35). The map was the result of a survey carried out at Dover Heights with the 9-Yagi 100 MHz antenna on its equatorial mount (Fig. 4.10). Bolton and Westfold (1950: 19) pointed out that their survey and those conducted by Hey et al. at 64 MHz and Reber at 160 MHz collectively provided “… a complete picture of the noise distribution over the whole celestial sphere … [which] will undoubtedly be of value in studying correlation between radio-frequency and optical data and possibly in deducing the form of the Galaxy.” The Sydney 100 MHz map showed that the strongest emission derives from the region of the Galactic Plane, mainly between Galactic longitudes 270° and 20°.

Although Bernie Mills and his Fleurs colleagues were primarily interested in discrete sources, their 85.5 MHz Mills Cross survey of the mid-1950s did provide data on overall Galactic background emission. Mills (1959a: 431) was able to identify two discrete types of emission:

(1) an extensive component of small axial ratio, displaying only moderate concentration toward the plane of the Milky Way and the galactic centre, which is here called the corona; [and] (2) a much brighter component, closely confined to the Galactic Plane and concentrated in the inner regions of the Galaxy (the disk) …

Fig. 4.36 shows the general distribution of the coronal emission, with the hatched region indicating the Galactic Plane component. Mills was not able to ascertain whether the non-uniformities in the coronal emission were primarily Galactic or extragalactic in origin.

The first comprehensive survey made by RP scientists at a much higher frequency was carried out by Jack Piddington and Gil Trent at 600 MHz using the 11 m dish at Potts Hill. This radio telescope, which had a 3.3° beam, was used in 1954 to survey Galactic emission between declinations –90° and +51°. This was one of the most comprehensive surveys conducted since Reber’s pioneering efforts, and Piddington was particularly proud of it. Piddington and Trent found – as with previous ‘all-sky’ surveys – that emission was concentrated along the Galactic Plane, where a succession of discrete sources is apparent (see Fig. 4.37). Piddington and Trent (1956a: 82) thought it likely that most of these discrete sources “… are thermally emitting clouds of ionised hydrogen.”

It was only with the advent of the 19.7 MHz Shain Cross at Fleurs that RP radio astronomers were finally able to gain an insight into the overall distribution of Galactic emission at low frequencies. Between 1956 and 1960, Alex Shain carried out an ambitious sky survey, using this pencil-beam radio telescope in ‘scanning mode’ to obtain records at five declinations separated by 0.5° to 0.75° (depending upon the zenith distance of the observing region). Scans through the Galactic Plane showed distinct dips rather than the peaks that are typically exhibited at higher frequencies, indicating that “… absorption of 19.7 Mc/s radiation is occurring in a band of HII regions near the galactic plane.” (Shain, 1957: 198). The 19.7 MHz map that Shain et al. (1961) published (Fig. 4.38) covers a strip of the Milky Way about 10° wide extending either side of the Galactic Centre, and it confirms the general coronal–Galactic Plane dichotomy noted by Mills et al. at 85.5 MHz.

Only with the advent of the Parkes Radio Telescope in 1961 would it be possible to complete more detailed all-sky surveys over a range of frequencies.

4.5 Distribution of Neutral Hydrogen

In 1944, in the depths of WWII, the Dutch astronomer Henk van de Hulst (1918–2000) predicted that neutral hydrogen in our Galaxy could be responsible for a radio emission line at 21 cm (or 1420 MHz) (Fig. 4.39). Although the Dutch were preparing to search for this line in 1951, it was in fact an American pair, Harold (‘Doc’) Ewen (1922–2015) and Edward Purcell (1912–1997), who made the discovery (Fig. 4.40). Frank Kerr (Biobox 4.5), destined to be Australia’s foremost H-line researcher, has fond memories of that event. At the time, he was at Harvard College Observatory broadening his knowledge of astronomy:

I happened to be there on the famous day (March 25, 1951) when H.I. (‘Doc’) Ewen and E.M. Purcell first detected the 21-cm line from neutral hydrogen in interstellar material in the Galaxy. H.C. van de Hulst, one of the pioneering Dutch radio astronomers, was also by chance at Harvard at that time. Purcell called us all together on the morning after that overnight discovery. Cautious scientist that he was, he wanted to see confirmation of the detection before publishing the result. He suggested that van de Hulst and I should cable our respective institutions in Holland and Australia to report the discovery and ask whether early confirmation would be possible. (Kerr, 1984: 137–138).

Biobox 4.5: Frank Kerr

Frank John Kerr (Fig. 4.41) was born on 8 January 1918 while his parents were in England (as part of the war effort), and the family returned to Australia in 1919. He studied physics and mathematics at the University of Melbourne, graduating with BSc and MSc degrees in 1938 and 1940 respectively (Kerr, 1986). WWII halted his plans to sit for a PhD at Cambridge, but in 1962 he was awarded a DSc by the University of Melbourne.

In 1940 Kerr joined the Radiophysics Lab and specialised in designing radar receiver components, as well as studying radio wave propagation in the Earth’s atmosphere. The latter led Frank to a moon-bounce experiment in 1947–48, and then spending a year at Harvard University in 1950–51 to learn more astronomy. Upon the discovery of the 21 cm spectral line of neutral hydrogen in 1951 (coincidentally at Harvard), Frank led Australia’s effort in this branch of radio astronomy, carrying out extensive observations of the Magellanic Clouds and our Milky Way with an 11 m dish at Potts Hill. The Australian data were combined with a similar effort by the Dutch group in Leiden to enable a first look at the spiral structure of the entire Milky Way. In the late 1950s he was part of the team that developed the Parkes Radio Telescope, which he then used for yet more detailed mapping of hydrogen in the southern Milky Way. His research projects always emphasised international cooperation and attention to optical astronomy results, as much as to those on the radio side.

In 1966 Kerr left Sydney to join the Faculty of Astronomy at the University of Maryland. Frank continued with further studies of the dynamics and structure of the Galaxy, as well as many other projects, such as studying details of the Galactic Centre by observing its fortuitous occultations by the Moon in the late 1960s. Over two decades he trained and inspired numerous graduate students in radio astronomy (including the third author WTS of this book). Frank Kerr died in Maryland on 15 September 2000. For further details see Sullivan (1988) and Westerhout (2000).

Alexander (Lex) Muller (1923–2004) and Jan Oort (1900–1992) in Holland quickly provided confirmation, but in Australia the challenge was considerable. Although Joe Pawsey and others had been aware of van de Hulst’s paper, there had been no attempt to follow it up. Bernie Mills (1976) thought that this was because

… no one really felt strongly enough about it to put the work into what we felt, or I felt, might be a rather speculative sort of thing. There was no certainty of positive results. There were lots of other things to be done … [and the H-line] appeared rather forbidding. One knew one had to get right down to the absolute maximum theoretical sensitivity because the thing was going to be faint … it wasn’t clear that it would be above the detectable limit. So it was a speculative project from that point of view.

Since no preparatory work had been done, Joe Pawsey asked both Chris Christiansen and Jim Hindman to construct H-line receivers. At first they worked independently and unbeknown to each other in adjacent instrument huts at Potts Hill located near the old Georges Heights radar antenna (see Fig. 2.42). Each RP radio astronomer made significant progress before discovering what the other was up to, and they then decided to combine their efforts.

The result was that in just six short but hectic weeks they were able to cobble together a primitive H-line receiver, which “… was held together with string and sealing wax … [and] kept going through shear will power.” (People magazine, 1954). At a much later date Christiansen described this receiver as “… the most terrible piece of equipment I’ve ever seen in all my life … It was a monster …” (Christiansen, 1978). Nevertheless, he and Hindman attached the ‘monster’ to the ex-Georges Heights radar antenna, and succeeded in detecting the hydrogen line. Christiansen (1978) proudly remembers that “The first time I saw the line I’d [nearly] given up. I thought the gear would never work. And then I went to sleep and came back and there was a beautiful curve sitting up on the chart.” (see Wendt et al., 2008 for more detail on these early 21 cm observations).

Ewen and Purcell’s paper appeared in the 1 September 1951 issue of Nature. It was immediately followed by one penned by the Dutch radio astronomers and by a note dated 12 July, hurriedly composed by Joe Pawsey on behalf of the RP team, and cabled from Australia:

Referring to Professor Purcell’s letter of June 14 announcing the discovery of hyperfine structure of the hydrogen line in galactic radio spectrum, confirmation of this has been obtained by Christiansen and Hindman, of the Radio Physics Laboratory, Commonwealth Scientific and Industrial Research Organization, using a narrow-beam aerial. Intensity and line-width are of same order as reported, and observations near declination 20°S. show similar extent about galactic equator. (Pawsey, 1951).

Thus began Australia’s assault on the H-line. For further details of the history of all aspects of 21-cm hydrogen-line research (up until 1953 and around the world) see Sullivan (2009).

During mid-1951 Christiansen and Hindman carried out exploratory H-line observations at Potts Hill, reporting their initial results in the Australian Journal of Scientific Research in 1952. They produced an isophote map of H-line emission extending over 270 degrees of Galactic longitude (including the Galactic Centre) along the Galactic Plane and from l = +40° to –50° (Fig. 4.42), and concluded that “… the source of line radiation occupies roughly the same part of the sky as does the visible Milky Way. Hence it may be assumed that the hydrogen is concentrated near the equatorial plane of the Galaxy.” (Christiansen and Hindman, 1952: 454–455). The existence of double line profiles over a considerable range of Galactic longitudes was interpreted as evidence of spiral arms in our Galaxy. These same authors also prepared a simple two-page summary of their findings for an international astronomical audience, and this was published in a 1952 issue of Observatory. When quizzed about these conclusions in 1976, Christiansen had to admit that at the time “We were just flapping around in the dark”, but the isophote plot “… was a really useful map because for many, many years radio astronomers used that map to find where the hydrogen was.” (Christiansen, 1978). Further details of this early research with the ex-Georges Heights WWII experimental radar antenna are included in Orchiston and Wendt (2017).

As an interesting aside, in 1952 Doc Ewen came to Australia to attend the Sydney URSI Congress (Fig. 4.43) to see the RP H-line receiver for himself. Christiansen (1978) later recalled how Ewen wanted “... to see how these damn Australians did in three weeks what took [other] people eighteen months to do. And when he saw the gear he just about passed out.”

In this same year Paul Wild (1952) (Fig. 2.46) published a seminal paper in the prestigious Astrophysical Journal on ways in which H-line investigations could be used to address some important astrophysical issues (see Wendt, 2011). As Kerr, Hindman and Robinson (1954: 297) were later to remark, the discovery of this line “… opened up new possibilities in astronomical exploration.” In particular, H-line radiation could penetrate clouds of interstellar dust and reveal the distribution of hydrogen gas, while frequency displacements of the line provided invaluable information about the motion of the emitting gas. And where independent measures of distance were available, the three-dimensional structure of the gas emitting regions could be deduced. The H-line was indeed an invaluable new research tool for astronomers.

After Christiansen and Hindman completed their pioneering 1951 study, Christiansen turned his attention to solar radio emission and innovative new instrumentation (his solar grating arrays) and it was left to Hindman, Kerr and young Brian Robinson (1930–2004; Biobox 4.6) to take the H-line work further. This they did in 1953 after construction of the 11 m Potts Hill dish and what Kerr (1984: 139) has described as “… the world’s first “multi-channel” receiver, which had all of four 40 kc/s channels!” (Fig. 2.44). They began by making the first H-line observations of extragalactic objects, in this case the Large and Small Magellanic Clouds, and published their results in the Australian Journal of Physics in 1954. In this important paper they showed that the neutral hydrogen extended well beyond the optical boundaries of each Magellanic Cloud (Fig. 4.45). They also showed that the total masses of neutral hydrogen in the Large and Small Clouds were about 600 and 400 million solar masses respectively, that the ratio of dust to gas in the two Clouds was very different and that both Clouds were rotating. A summary of the preliminary findings of this study was presented at a meeting of the American Astronomical Society in August 1953, and subsequently published in the Astronomical Journal.

Biobox 4.6: Brian Robinson

Brian John Robinson (Fig. 4.44) was born in Melbourne on 4 November 1930 and died at Bonnells Bay, NSW, on 22 July 2004. After graduating with a BSc (First Class Honours and the University Medal) in Physics from the University of Sydney in 1952, he completed an MSc at the same university, and then joined the Radiophysics Lab in 1953. The following year, Brian moved to Trinity College, Cambridge, to study for a PhD under Jack Ratcliffe, graduating in 1958.

Brian then spent four years at Leiden Observatory working mainly on parametric amplifiers, and brought this new technology back to the Parkes Radio Telescope in 1962. His early research was on H-line and OH-line emission, but then progressed to a study of a range of new interstellar molecules. Pulsars were another interest and one of his 1968 chart records was reproduced on the Australian $50 bank note (see Fig. 5.8). Brian was also involved in the development of new facilities, including the millimetre radio telescope at the ATNF site in Marsfield, NSW, and he played a key role in the design of the Australia Telescope Compact Array.

Brian moved quickly through the CSIRO ranks, rising to Chief Research Scientist before his retirement in 1992. He was active in the IAU and from 1975 to 1979 he chaired the Australian National Committee for Radio Science. In 1974 he was awarded the Walter Burfitt Prize by the Royal Society of New South Wales for his outstanding contributions to the field of radio physics. For further details see Whiteoak and Sim (2006).

Kerr and the Australian-based French-born astronomer Gérard de Vaucouleurs (1918–1995) followed up these papers with a study of “… the three-dimensional distribution of gas density and rotational motion” in the Magellanic Clouds (Kerr and de Vaucouleurs, 1955: 509). After first considering the roles of the Sun’s motion in the Galaxy and for Galactic rotation, they examined the evidence for rotation in each of the Clouds and found it convincing (see Fig. 4.46). Radio data supported the view that the Large Magellanic Cloud is a flattened system that is tilted by at least 65° relative to our line of sight. The tilt angle of the Small Magellanic Cloud is only about 30°, and at 1420 MHz it has “… a large prominence, or wing, extending towards the Large Cloud.” (Kerr and de Vaucouleurs, 1955: 515). This unusual feature is now known to be part of the ‘Magellanic Stream’.

One of the most fascinating H-line studies carried out was a collaboration between the Sydney and Dutch groups to map the locations of the spiral arms in our Galaxy. After publication of some preliminary results of the Sydney study by Kerr and visiting American radio astronomer Martha (‘Patty’) Stahr Carpenter (1920–2013; Fig. 4.47), the seminal paper in Nature by Kerr, Hindman and Carpenter (1957) provided impressive pictorial evidence of the spiral nature of our Galaxy. As Fig. 4.48 illustrates, the Potts Hill observations provided evidence of at least four major spiral arms (the so-called Scutum-Norma, Sagittarius, Orion and Perseus Arms). The Sydney observations also provided interesting information on the distribution of hydrogen gas in the plane of the Galaxy. Meanwhile, Kerr and Dutch colleagues, Jan Oort and Gart Westerhout, took the combined Sydney and Leiden observations further in a paper published in 1958 in the Monthly Notices of the Royal Astronomical Society. Of particular interest is their map of the overall distribution of neutral hydrogen in the plane of the Galaxy (Fig. 4.49), which shows great irregularity and does not indicate the spiral arms as prominently as in Fig. 4.48. Nonetheless, Oort, Kerr and Westerhout (1958) concluded that our Galaxy probably belongs to Edwin Hubble’s class of spiral galaxies called Sb.

Frank Kerr followed up his initial Galactic H-line survey with a detailed study of hydrogen emission from the Southern Milky Way, carried out in collaboration with RP colleagues Jim Hindman and Colin Gum (1924–1960). When coupled with a similar northern study, this was used by Gum, Kerr and Westerhout, and by Gum and Pawsey to document a new Galactic Co-ordinate System adopted by the International Astronomical Union in March 1959. As Dutch astronomer Adriaan Blaauw (1914–2010), along with Gum, Pawsey and Westerhout outlined, Australian radio astronomy played a crucial role in this important study (Blaauw et al., 1960).

Subsequently, Kerr carried out an analysis of the differences that typified the Sydney and Leiden H-line data, and he and Westerhout penned a chapter titled “Distribution of Interstellar Hydrogen” for Volume 5 of the definitive series Stars and Stellar Systems (Volume 5) (Kerr and Westerhout, 1965). The Australian radio astronomers certainly had made ‘good mileage’ out of their pioneering Potts Hill observations.

Further H-line progress was only possible with the advent of the Murraybank field station (Fig. 2.68) in 1956, and completion of a 48-channel receiver and a 6.4 m alt-azimuth-mounted dish. The sophistication of the new receiver, and the ability to access designated regions of sky – instead of relying on the Earth’s rotation as at Potts Hill – revitalised Australian H-line work. Back in 1954, Joe Pawsey foreshadowed the importance of these developments:

The crying need here [with H-line studies] is for first-class equipment. The choice of observations is still easy in this unexploited field. The current instrumental deficiency is a receiver giving an instantaneous profile. When this is satisfied there will be a bread-and-butter survey problem extending over years ... (Pawsey, 1954).

The initial plan was to acquire an 18 m dish and to use this for high-resolution H-line work, but when this antenna eventually arrived (Fig. 2.66) it ended up at Fleurs, not Murraybank, and was used for continuum studies (for details, see Orchiston and Mathewson, 2009).

Instead, Dick McGee and John Murray (1924–2020; Fig. 2.69) had to make use of the very much smaller 6.4 m antenna. In an initial test of the new dish and 48-channel receiver and its large-scale survey potential, they investigated the distribution of neutral hydrogen in the Taurus–Orion region. The new receiver allowed them to obtain an individual H-line profile in just two minutes, and to generate composite profiles by adding these. More than 3500 profiles were obtained and some 3500 square degrees of sky were surveyed. The level of emission suggested that a large single neutral hydrogen cloud or an association of connected clouds spans the Taurus–Orion region and that this is rotating as part of the general structure of the Galaxy (McGee and Murray, 1959).

Dick McGee, John Murray and research assistant Janice Milton followed up this study with a survey of the distribution of neutral hydrogen over the whole sky visible from Sydney. Meridian transit observations were made at 1° intervals between declinations –90° and +42°, and more than 95,000 profiles were obtained. McGee and Murray found evidence that in general the gas was stratified parallel to the Galactic Plane, although there were some regions of excess density. Meanwhile, in our region of the Galaxy, they found that

… hydrogen is flowing away from the Sun at about 6 km/s in the direction of the galactic centre and anticentre in low and medium latitudes and is streaming in from above and below in latitudes |b^I| = 90° to |b^I| ≈ 40° at about 6 km/s. (McGee and Murray, 1961: 278).

In a follow-up paper McGee, Murray and Milton (1963) reported on the detailed distribution of low velocity gas, and produced a composite map (Fig. 4.50) highlighting the gas concentration along the Galactic Plane. In a third and final paper in the series, McGee and Milton examined the distribution and intensity of hydrogen gas at higher radial velocities, finding that it was concentrated in a number of massive spiral arms of our Galaxy.

One of the problems encountered with the H-line receiver at Murraybank was the enormous quantity of data obtained in a relatively short time, which prompted the development of a digital data-recording system that ultimately saw extensive use with the 64 m Parkes Radio Telescope. However, this innovative system was first trialed at Murraybank during a low resolution H-line survey of the Magellanic Clouds. The digital recording and data handling system successfully converted 250 hours of observations to printed profiles in just eight hours of computer time (Hindman et al., 1963a, 1963b), but more than this, the survey reinforced the earlier finding of Kerr, Hindman and Robinson (1954) that an extensive gaseous envelope enclosed both Magellanic Clouds. The total mass of hydrogen in the two Clouds derived in the earlier study was also confirmed. A new discovery was the detection of a tenuous ‘bridge’ of hydrogen gas between the Large and Small Magellanic Clouds (see Fig. 4.51). And a further discovery, associated with the Small Magellanic Cloud, was the existence of double peaks over a wide area, suggesting the possibility that this Cloud consists of two quite separate masses of gas. With the closure of the Murraybank field station, the H-line work transferred easily and automatically to Parkes. For details of the international contribution that the Murraybank field station made to international radio astronomy, see Wendt et al. (2011a).

The 1420 MHz (21 cm) hydrogen line was the only radio line observed by the RP radio astronomers at the field stations. However, Gordon Stanley and visiting Fulbright fellow Robert Price (1929–2008) did conduct an unsuccessful search for a postulated 327 MHz deuterium line in 1954 with the 24.4 m ‘hole-in-the-ground’ antenna at Dover Heights (deuterium is an isotope of hydrogen with a nucleus consisting of one proton and one neutron). They had hoped that they might see this in absorption against the Sagittarius A and Centaurus A sources. At the time, they did not publish their ‘null result’, but when Russian colleagues reported a detection at flux levels far in excess of the upper limit they had set, they were forced into print with a rebuttal (Stanley and Price, 1956). The elusive deuterium line was eventually detected in 2007 by a group at the MIT Haystack Observatory in Massachusetts.

4.6 Radio Emission from Stars

Once the true nature of Taurus A, Centaurus A, Virgo A and other discrete sources became known in the late 1940s, the search was on for genuine ‘radio stars’. Two scientists were involved in making the first detections of radio emission from stars other than the Sun: Bernard Lovell (1913–2012; Fig. 1.40) at Jodrell Bank and Bruce Slee at RP. Both began by selecting the dMe flare stars as their targets, and to Slee the reasons were obvious:

Detection of radio emission from flare stars might lead to several important new fields of research. For example, although probably 80 percent of all main sequence stars are M dwarfs, we know little about the physical conditions in their atmospheres. Studies of radio bursts – which could be generated in these stars’ coronas, if indeed they have extensive atmospheres like the Sun – would be valuable in this connection.

Again, statistics of flare-star bursts may tell whether stars in general possess “star spot” cycles analogous to the 11-year sun-spot cycle. Finally, radio studies could show whether flare stars eject high-energy particles in sufficient amount to contribute appreciably to cosmic rays and, indirectly, to the radio emission of the Milky Way galaxy. (Slee, Higgins and Patston, 1963: 83).

From September 1960 to May 1962, Slee and Charlie Higgins carried out intermittent monitoring of four different flare stars: UV Ceti, Proxima Centauri, V371 Orionis and V1216 Sagittarii. Most of their observations were made at Fleurs with the N–S arms of the 85.5 MHz and 19.7 MHz Mills and Shain Crosses (Figs 2.59 and 2.63), but some observations were also carried out (at 400 and 1500 MHz) with the new Parkes Radio Telescope. Teams of amateur astronomers from Queensland, New South Wales and Victoria were enlisted to simultaneously observe during parts of the radio monitoring programs. During 1960–61 the first author WO of this book led one of these programs, using an historic 6-in Grubb refractor at Sydney Observatory (see Orchiston, 2016b: 5).

Although two strong optical flares were recorded, neither was accompanied by 85.5 MHz emission, but within a few minutes of each of three weaker flares

… radio deflections were recorded … In all three cases … the 3.5-meter [85.5 MHz] bursts were small, but during the possible flare of UV Ceti on November 13, 1961, concurrent observations at 15 metres [19.7 MHz] revealed an extremely intense group of bursts, about four minutes after a similar but much weaker group on 3.5 metres. (Slee, Higgins and Patston, 1963: 85).

In addition, radio bursts were also recorded at 85.5 MHz on five different occasions when there was no optical monitoring.

These landmark Fleurs observations and similar results reported at much the same time by Lovell at Jodrell Bank and colleagues at the Smithsonian Astrophysical Observatory in Massachusetts (Lovell et al., 1963) marked the initiation of stellar radio astronomy. Orchiston (2004: 36–46) has documented how Bruce Slee went on to build a reputation as a world authority on radio emission from flare stars and other types of ‘active stars’ (e.g. see Fig. 4.52).

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Authors and Affiliations

Centre for Astrophysics University of Southern Queensland, Toowoomba, QLD, Australia
Wayne Orchiston
National Astronomical Research Institute of Thailand, Mae Taeng, Chiang Mai, Thailand
Wayne Orchiston
School of Physics, University of Melbourne, Melbourne, VIC, Australia
Peter Robertson
Astronomy Department, University of Washington, Seattle, WA, USA
Woodruff T. Sullivan III

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Wayne Orchiston
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Peter Robertson
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Woodruff T. Sullivan III
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Orchiston, W., Robertson, P., Sullivan III, W.T. (2021). Expanding Horizons – The Milky Way and Beyond. In: Golden Years of Australian Radio Astronomy. Historical & Cultural Astronomy. Springer, Cham. https://doi.org/10.1007/978-3-319-91843-3_4

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DOI: https://doi.org/10.1007/978-3-319-91843-3_4
Published: 20 January 2022
Publisher Name: Springer, Cham
Print ISBN: 978-3-319-91841-9
Online ISBN: 978-3-319-91843-3
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Expanding Horizons – The Milky Way and Beyond

Abstract

4.1 Discovery of the First Discrete Radio Sources

Biobox 4.1: Bruce Slee

Biobox 4.2: Gordon Stanley

4.2 From Dover Heights to Fleurs

Biobox 4.3: Alec Little

4.3 Radio Emission from the Galactic Centre

Biobox 4.4: Dick McGee

4.4 Surveys of the Background Radio Emission

4.5 Distribution of Neutral Hydrogen

Biobox 4.5: Frank Kerr

Biobox 4.6: Brian Robinson

4.6 Radio Emission from Stars

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