«Bull. Astr. Soc. India (2013) 41, 1–17 The discovery of quasars K. I. Kellermann National Radio Astronomy Observatory, 520 Edgemont Road, ...»
Bull. Astr. Soc. India (2013) 41, 1–17
The discovery of quasars
K. I. Kellermann
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22901, USA
Received 2013 February 01; accepted 2013 March 26
Abstract. Although the extragalactic nature of quasars was discussed as early as 1960,
it was rejected largely because of preconceived ideas about what appeared to be an unrealistically high radio and optical luminosity. Following the 1962 occultations of the
strong radio source 3C 273 at Parkes, and the subsequent identiﬁcation with an apparent stellar object, Maarten Schmidt recognized that the relatively simple hydrogen line Balmer series spectrum implied a redshift of 0.16. Successive radio and optical measurements quickly led to the identiﬁcation of other quasars with increasingly large redshifts and the general, although for some decades not universal, acceptance of quasars as being by far the most distant and the most luminous objects in the Universe.
Arguments for a more local population continued for at least several decades, fueled in part by a greater willingness to accept the unclear new physics needed to interpret the large observed redshifts rather than the extreme luminosities and energies implied by the cosmological interpretation of the redshifts.
Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was ﬁrst catalogued in 1959 and the magnitude 13 optical counterpart was observed at least as early as 1887. Since 1960, much fainter optical counterparts were being routinely identiﬁed using accurate radio interferometer positions, measured primarily at the Caltech Owens Valley Radio Observatory. However, 3C 273 eluded identiﬁcation until the series of lunar occultation observations led by Cyril Hazard.
Subsequent attempts to classify quasars into numerous sub-categories based on their observed optical, radio, IR and high energy properties have perhaps led to more confusion than clarity. However, quasars and the broader class of AGN are now a fundamental part of astrophysics and cosmology. They were the basis for the recognition of supermassive black holes in galactic nuclei, which are intimately tied to the formation and evolution of stars and galaxies.
Keywords : quasars: general – quasars: emission lines – radio continuum: stars – galaxies: distances and redshifts – galaxies: active – occultations – history and philosophy of astronomy 2 K. I. Kellermann
1. Historical background The year 2013 marks the ﬁftieth anniversary of the discovery of quasars. It was on 5 February, 1963, that Maarten Schmidt at Caltech recognized that the spectrum of the magnitude 13 apparently stellar object identiﬁed with the radio source 3C 273 could be most straightforwardly interpreted by a redshift of 0.16. Subsequent work by Schmidt and others led to increasingly large measured redshifts and the recognition of the broad class of active galactic nuclei (AGN) of which quasars occupy the high luminosity end. Schmidt’s discovery changed extragalactic astronomy in a fundamental way. Measured redshifts quickly went from about 0.5 to more than 2 in only a few years, making possible a new range of cosmological studies, as well as the realization that supermassive black holes which power radio galaxies and quasars play a prominent role in the evolution of galaxies. But the path to this understanding was a slow, tortuous one, with missed turns that could have earlier deﬁned the nature of quasars.
The events leading up to the recognition of quasars as the extremely luminous nuclei of distant galaxies go back much earlier than 1963; indeed, one wonders why the extragalactic nature of quasars was not recognized well before 1963, and why 3C 273, which is the seventh brightest radio source in the northern sky away from the Galactic plane, was not identiﬁed at least one or two years earlier.
In the remainder of this section I review the early arguments and evidence for powerful activity in the nuclei of galaxies. In Section 2, I brieﬂy review the status of extragalactic radio astronomy prior to the identiﬁcation of 3C 48, and in Section 3, the identiﬁcation of 3C 48, which might have been the ﬁrst discovered quasar, but was unrecognized as such until the work on 3C 273 three years later as described in Section 4. In Section 5 I return to the issues surrounding 3C 48, and in Sections 6 and 7 the implications for cosmology and the arguments for non-cosmological interpretations of quasar redshifts. Sections 8 and 9 describe the discovery of radio quiet quasars and the proliferation of quasar categories and classiﬁcations. Finally, in Section 10, I summarize the history and remaining questions surrounding the discovery of quasars.
Probably the ﬁrst person to note the enhanced activity in the nucleus of a galaxy was Edward Fath (1908) who reported on the nuclear emission line spectrum of NGC 1068. Later observations of strong nuclear emission lines by V. Slipher (1917), Edwin Hubble (1926), Milton Humason (1932), and Nick Mayall (1934, 1939) led Carl Seyfert (1943) to his now famous study of the enhanced activity in the nuclei of six galaxies (or as he called them, ‘extragalactic nebulae’).
Seyfert, as well as his predecessors, commented on the similarity with the emission line spectrum of planetary and other gaseous nebulae and that the lines are apparently Doppler broadened.
There is no evidence that he ever continued this work, but nevertheless galaxies containing a stellar nucleus with strong broad emission (including forbidden) lines have become known as ‘Seyfert Galaxies’. Unfortunately, Seyfert died in an automobile accident in 1960. Before his death, he served three years on the board of Associated Universities Inc. during the critical period when AUI was overseeing the early years of NRAO. It interesting to speculate whether if Seyfert had lived, his association with the radio astronomers at NRAO might have led to an earlier appreciation of the relationship between radio emission and nuclear activity.
The discovery of quasars However, it was Victor Ambartsumian who mainly championed the idea that something special was going on in the nuclei of galaxies. At the 1958 Solvey Conference, Ambartsumian (1958) proposed ‘a radical change in the conception on the nuclei of galaxies’, saying ‘apparently we must reject the idea that the nuclei of galaxies is composed of stars alone’. He went on to conclude that, ‘large masses of prestellar matter are present in nuclei’.
Even earlier, Sir James Jeans (1929) wrote:
‘The type of conjecture which presents itself, somewhat insistently, is that the centres of the nebulae are of the nature of “singular points”, at which matter is poured into our universe from some other and entirely spatial dimension, so that to a denizen of our universe, they appear as points at which matter is being continually created.’ In a prescient paper, Hoyle and Fowler (1963) considered ‘the existence at the very centre of galaxies of a stellar-type object of large mass... in which angular momentum is transferred from the central star to a surrounding disk of gas’.
When discrete sources of radio emission were discovered, they were ﬁrst thought to be due to Galactic stars. Both, Karl Jansky and Grote Reber had shown that the diﬀuse radio emission was associated with the Milky Way, and since the Milky Way is composed of stars, it seemed natural that the discrete radio sources were likely connected with stars. Indeed for many years they were called ‘radio stars’.
The ﬁrst hint that at least some radio sources might be extragalactic came from a series of observations made by John Bolton, Gordon Stanley, and Bruce Slee (1949) using cliﬀ interferometers in Australia and New Zealand. After months of painstaking observations, Bolton and his colleagues succeeded in measuring the positions of three strong radio sources with an accuracy better than half a degree. For the ﬁrst time it was possible to associate radio sources with known optical objects. Taurus A, Centaurus A, and Virgo A were identiﬁed with the Crab Nebula, NGC 5128 and M87 respectively. NGC 5128, with its conspicuous dark lane, and M87, with its prominent jet, were well known to astronomers as peculiar galaxies. But, their 1949 paper, ‘Positions of Three Discrete Sources of Galactic Radio Frequency Radiation’, which was published in Nature mostly discussed the nature of the Crab Nebula. In a few paragraphs near the end of their paper, they commented, ‘NGC 5128 and NGC 4486 (M87) have not been resolved into stars, so there is little direct evidence that they are true galaxies. If the identiﬁcation of the radio sources are accepted, it would indicate that they are [within our own Galaxy]’. As implied by the title, Bolton, Stanley, and Slee dismissed the extragalactic nature of both Centaurus A and M87. When asked many years later why he didn’t recognize that he had discovered the ﬁrst radio galaxies, Bolton responded that he knew they were extragalactic, but that he also realized that the corresponding radio luminosity would be orders of magnitude greater than that of our Galaxy and that he was concerned that in view of this apparent extraordinary luminosity, a conservative Nature 4 K. I. Kellermann referee might hold up publication the paper. However, in a 1989 talk, Bolton (1990) commented that their 1949 paper marked the beginning of extragalactic radio astronomy. Nevertheless, for the next few years the nature of discrete radio sources remained controversial within the radio astronomy community, and many workers, particularly at the Cambridge University Cavendish Laboratory, continued to refer to radio stars.
Following the identiﬁcation of Cygnus A with a magnitude 18 galaxy at z = 0.06, by Walter Baade and Rudolph Minkowski (1954), it became widely appreciated that the high latitude radio sources were in fact very powerful ‘radio galaxies’, and that the fainter radio sources might be at much larger redshifts, even beyond the limits of the most powerful optical telescopes such as the Palomar 200 inch. In a footnote to their paper, Baade and Minkowski confess that Cygnus A had been previously identiﬁed with the same galaxy by Mills and Thomas (1951) and by Dewhurst (1951), but at the time Minkowski was not willing to accept the identiﬁcation with such a faint and distant nebula. Over the next ﬁve years, many other radio galaxies were recognized based on more accurate radio positions measured at Cambridge, and starting in 1960, with the Caltech interferometer, which John Bolton had built speciﬁcally to obtain more accurate radio source positions in order to enable more optical identiﬁcations. Recognizing that radio galaxies were characteristically the brightest galaxy in a cluster, it became clear to many that the search for distant galaxies needed to address the outstanding cosmological problems of the day should therefore concentrate on galaxies identiﬁed with radio sources. Moreover, it was naturally assumed that the smaller radio sources were most likely to be the more distant, so emphasis was given to the smallest radio sources, whose dimensions were determined with the long baseline radio linked interferometers at Jodrell Bank (Allen et al. 1960; Allen et al. 1962) and with the Caltech OVRO (Owens Valley Radio Observatory) interferometer.
Much of this work was carried out within a collaboration of scientists at Caltech and at the Mount Wilson and Palomar Observatories. John Bolton, Tom Matthews, Alan Moﬀet, Dick Read, and Per Maltby at the Caltech OVRO provided accurate radio positions, angular sizes, and optical identiﬁcations based on inspection of the 48 inch Schmidt prints and plates. At the Mt. Wilson and Palomar Observatories, Baade, Minkowski, and Allan Sandage teamed up with the Caltech radio astronomers to obtain 200 inch photographs and spectra. At Caltech, Jesse Greenstein, Guido Munch and, after Minkowski’s retirement in 1960, Maarten Schmidt provided spectroscopic follow-up to determine the redshifts of radio galaxies.
This program had a dramatic success, when, using the 200 inch telescope during his last observing session before retiring, Minkowski (1960) determined a redshift of 0.46 of the 20.5 magnitude galaxy which was identiﬁed by Matthews and Bolton with 3C 295. This made 3C 295 by far the largest known redshift. Although previous to Minkowski’s observation, the largest measured spectroscopic redshift was less than 0.2, curiously an unrelated foreground galaxy located only a few arcmin from 3C 295 was observed by Minkowski to have a redshift of 0.24, making it the second largest known redshift at the time. Yet, it would be another 15 years before a galaxy redshift greater than that of 3C 295 would be measured. Interestingly, 3C 295 was targeted not because of any special properties, but only because it was at a high declination, where The discovery of quasars an accurate declination could be measured with the OVRO interferometer, which at that time had only an East-West baseline.
By late 1960, it was widely accepted that radio sources located away from the Galactic plane were powerful distant radio galaxies (e.g., Bolton 1960). But, Bolton’s Caltech colleague, Jesse Greenstein, a world expert on exotic stars, was said to have oﬀered a bottle of the best Scotch whisky to whoever found the ﬁrst true radio star. Meanwhile, in the quest to ﬁnd more distant galaxies, the Caltech identiﬁcation program concentrated on small diameter sources selected from the early OVRO interferometer observations and from unpublished long baseline interferometer observations at Jodrell Bank (Allen et al. 1962).