Early Australian Optical and Radio Observations of Centaurus A

CSIRO PUBLISHING Publications of the Astronomical Society of Australia, 2010, 27, 402–430 www.publish.csiro.au/journals/pasa Early Australian Optical and Radio Observations of Centaurus A Peter RobertsonA,C, Glen CozensA, Wayne OrchistonA, Bruce SleeA,B, and Harry WendtA A Centre for Astronomy, James Cook University, Townsville, Qld 4811, Australia B CSIRO Astronomy and Space Sciences, PO Box 76, Epping, NSW 2121, Australia C Corresponding author. Email: [email protected] Received 2009 October 8, accepted 2009 December 8 Abstract: The discovery of the radio source Centaurus A and its optical counterpart NGC 5128 were important landmarks in the history of Australian astronomy. NGC 5128 was first observed in August 1826 by James Dunlop during a survey of southern objects at the Parramatta Observatory, west of the settlement at Sydney Cove. The observatory had been founded a few years earlier by Thomas Brisbane, the new governor of the British colony of New South Wales. Just over 120 years later, John Bolton, Gordon Stanley and Bruce Slee discovered the radio source Centaurus A at the Dover Heights field station in Sydney, operated by CSIRO’s Radiophysics Laboratory (the forerunner to CSIRO Astronomy and Space Sciences). This paper will describe this early historical work and summarize further studies of Centaurus A by other Radiophysics groups up to 1960. Keywords: Centaurus A — NGC 5128 — radio galaxies — history of Australian astronomy — Parramatta Observatory — Radiophysics Laboratory 1 Introduction different nature began at Dover Heights in Sydney, the site The radio source Centaurus A and its optical counterpart of a former wartime radar research station operated by the NGC 5128 comprise possibly the most important astro- Radiophysics Laboratory. John Bolton, Gordon Stanley nomical object in the history of Australian astronomy. and Bruce Slee used a novel technique known as sea Observations of the southern sky in Australia are as old as interferometry and were able to confirm earlier reports human settlement. There is a growing body of evidence from England that there was strong radio emission from a that the importance of astronomical knowledge to concentrated area in the constellation of Cygnus. As we see Aboriginal culture was far greater than previously thought in Section 3, by the end of 1947 the group at Dover Heights (see, e.g., Norris 2008). With the arrival of British set- had discovered several other point-like sources of radio tlement in 1788, a rudimentary observatory was estab- emission, including one in the Centaurus constellation. lished at Sydney Cove in the new colony of New South As the number of these new radio sources grew, the Wales, but it was used primarily for time-keeping and as a most important challenge was to determine the nature of navigational aid and did not produce any astronomical these sources and to see whether they could be identified observations of significance (see Haynes et al. 1996). with objects known to astronomers. As we see in Section 4, Astronomy as a science in Australia began with the Bolton and Stanley organized an expedition to New establishment of the Parramatta Observatory in 1822 by Zealand in mid-1948 where better observing conditions Sir Thomas Brisbane, the sixth governor of the British meant that the positions of the known radio sources could colony of New South Wales. Brisbane is best remembered be measured with greater accuracy. In mid-1949, just two today by the name of Australia’s third largest city, the years after the initial Cygnus detection, the Dover Heights capital of the state of Queensland. The observatory group published a paper with the new and more accurate carried out the first systematic survey of southern stars positions. For three of the sources they offered tentative and the catalogue was eventually published in 1835. In identifications with known optical objects, each of which 1826, Brisbane’s assistant, James Dunlop, carried out turned out to be correct. The Taurus A source coincided another survey of southern nebulae and star clusters. Over with the Crab Nebula and Virgo A with M87. The third 600 objects were catalogued, and among them was an source, Centaurus A, coincided with NGC 5128, the object later named NGC 5128. As we discuss in Section 2, nebula discovered by James Dunlop in 1826. Dunlop’s catalogue was published in England in 1828, 40 After 1949, the focus of the Dover Heights group was years after the establishment of the new colony, and was to build a series of increasingly sensitive radio telescopes the first astronomical paper published from Australia. to survey the sky. The number of known radio sources In mid-1947, just over 120 years after Dunlop began his soon ran into the hundreds. In Section 5 we describe survey, observations of the southern sky of a radically further study of Centaurus A at Dover Heights. Rather Ó Astronomical Society of Australia 2010 10.1071/AS09071 1323-3580/10/04402 Downloaded from https://www.cambridge.org/core. IP address: 170.106.202.8, on 30 Sep 2021 at 01:01:45, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1071/AS09071 Early Australian Optical and Radio Observations of Centaurus A 403 than a point-like source, this nearby galaxy was shown to have a very large angular size with a complex internal structure where the strength of the radio emission varies markedly over the area of the source. With the closure of the Dover Heights field station in 1954, other groups within the Radiophysics Laboratory studied Centaurus A at several other field stations operat- ing in the greater Sydney area. In Section 6 we describe this work up until 1960. 2 The Discovery of NGC 5128 2.1 Setting the Scene: The Parramatta Observatory The galaxy NGC 5128 was discovered by James Dunlop on 29 April 1826 at Parramatta, a small settlement ,20 km west of Sydney, in the British colony of New South Wales. (We use the contemporary spelling ‘Parramatta’ through- out this paper, rather than ‘Paramatta’ as it was then known.) Dunlop was born at Dalry, 35 km south-west of Glasgow, Scotland, on 31 October 1793. At age 14, he moved to nearby Beith where he lived with his father’s twin brother and worked in a thread factory. James was not well educated. According to one of his biographers, John Service (1890), ‘y hehadbeenashorttimeat school in Dalry, and when he went to Beith, he attended a night-school in the Strand y But, beyond these meagre opportunities for education, he received no scholastic Figure 1 Sir Thomas Makdougall Brisbane (1773–1860; portrait training whatsoever’. Dunlop had a ‘natural aptitude and by F. Schenck, ca. 1850, courtesy State Library of NSW). love [for mechanics and] when he was seventeen years of age, he was constructing lathes and telescopes and casting reflectors for himself’ (Service 1890; Wood After a 6-month voyage via Rio de Janeiro onboard the 1966; Saunders 1990). Royal George, Brisbane and his assistants arrived in Sydney In 1820, Dunlop was introduced to Sir Thomas Bris- in November 1821. Brisbane immediately arranged for the bane, a meeting that led to Dunlop accompanying Brisbane construction of the observatory next to Government House to Australia. Brisbane was also born in Scotland and at Parramatta, west of the main settlement at Sydney Cove. educated at the University of Edinburgh. He then joined Parramatta Observatory was completed within months (see the British Army where he rose to the rank of Lieutenant- Figure 2) and fitted out with Brisbane’s instruments and General (Heydon 1966; Saunders 2004). During the personal library. One of the principal instruments was a Peninsular War he commanded a brigade that was heavily 9.2 cm diameter transit telescope which would be used to engaged in battles in France. Brisbane was knighted in survey the southern sky. Other notable instruments included 1815 in recognition of his service. The Duke of Wellington a Reichenbach repeating circle and an 8.3 cm refracting recommended him to the Colonial Office for higher duties telescope (see Lomb 2004). and, in November 1820, Brisbane was offered the post of The observatory got off to a flying start when in June governor of the colony of New South Wales (see Figure 1). 1822, only a month after observations began, Dunlop and Aside from his military career, Brisbane was a fine Ru¨mker observed the return of Encke’s Comet. Dunlop example of a 19th century gentleman-scientist. He had a found the comet at the position calculated by Ru¨mker. keen interest in astronomy and in 1808 built an observa- After Halley’s Comet in 1758 this was only the second tory at his own expense in Ayrshire, south of Glasgow occasion the return of a comet as predicted had been (Morrison-Low 2004). For his work there he was made a observed and it confirmed that comets, like planets, Fellow of the Astronomical Society of London. On his obeyed Newton’s laws. The importance of the recovery appointment as Governor of New South Wales, Brisbane of the comet can be gauged by the immediate award of decided he would build another observatory to explore the 100 pounds to Ru¨mker by the Astronomical Society of largely unchartered southern skies. He knew he would be London (Bergman 1967). too busy with his own duties as governor, so he hired two Aside from Encke’s Comet, Ru¨mker and Dunlop assistants — again at his own expense. One was James embarked on an intense period of observing and catalo- Dunlop, for his talents as an instrument maker, and the guing the stars of the southern sky.

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