aa r X i v : . [ a s t r o - ph . I M ] D ec Extragalactic jets from every angleProceedings IAU Symposium No. 313, 2015F. Massaro, C.C. Cheung, E. Lopez, A. Siemiginowska, eds. c (cid:13) The Road to Quasars
K. I. Kellermann
National radio Astronomy Observatory520 Edgemont Rd, Charlottesville, VA 22903email: [email protected]
Abstract.
Although the extragalactic nature of 3C 48 and other quasi stellar radio sourceswas discussed as early as 1960 by John Bolton and others, it was rejected largely because ofpreconceived ideas about what appeared to be unrealistically high radio and optical luminosities.Not until the 1962 occultations of the strong radio source 3C 273 at Parkes, which led MaartenSchmidt to identify 3C 273 with an apparent stellar object at a redshift of 0.16, was the truenature understood. Successive radio and optical measurements quickly led to the identificationof other quasars with increasingly large redshifts and the general, although for some decadesnot universal, acceptance of quasars as the very luminous nuclei of galaxies.Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was firstcataloged in 1959 and the magnitude 13 optical counterpart was observed at least as early as1887. Since 1960, much fainter optical counterparts were being routinely identified using accurateradio interferometer positions which were measured primarily at the Caltech Owens ValleyRadio Observatory. However, 3C 273 eluded identification until the series of lunar occultationobservations led by Cyril Hazard. Although an accurate radio position had been obtained earlierwith the OVRO interferometer, inexplicably 3C 273 was initially misidentified with a faint galaxylocated about an arc minute away from the true quasar position.
Keywords. quasars, AGN.
1. Introduction
The discovery of radio galaxies and then quasars had a profound impact on develop-ment of astrophysics and cosmology during the later part of the 20th century. Suddenlyastronomers were faced with previously unimaginable luminosities coming from unimag-inably small volumes. Following the 1963 identification of 3C 273 at z=0.16, redshiftswere quickly extended to z ∼ times more luminous than that ofthe Crab Nebula, they concluded that NGC 5128 and NGC 4486 (M87) have not been resolved into stars, so there islittle direct evidence that they are true galaxies. If the identification of the radiosources are accepted, it would indicate that they are [within our own Galaxy].
They entitled their paper simply “Positions of Three Discrete Radio Sources of GalacticRadio Frequency Radiation.” John Bolton later explained that he really did understandthat M87 and NGC 5128 were extraordinarily luminous extragalactic radio sources, butthat he was concerned that that a conservative Nature referee might hold up publication(personal communication from Bolton to the author, August, 1989).The following years saw the identification of more radio galaxies, and the paradigmwhich had previously considered all discrete radio sources to be galactic stars quicklychanged to one where to most high latitude sources were assumed to be extragalactic. Theenergy requirements were exacerbated with the identification of Cygnus A, the secondstrongest radio source in the sky with a magnitude 18 galaxy which was at what wasthen considered a high redshift of 0.056 and a corresponding radio luminosity about 10 times more luminous than M87 or NGC 5128 (Baade and Minkowski 1954).By the end of the decade, many radio sources had been identified with optical counter-parts, but the redshift of Cygnus A remained the largest of any known radio galaxy. Typi-cally the optical counterparts of strong radio sources were elliptical galaxies Bolton (1960)that were the brightest member of a cluster. In 1960, Rudolph Minkowski (1960) iden-tified 3C 295 with a magnitude 20 galaxy at z=0.46, by far the largest known redshiftof any galaxy. 3C 295 is about ten times smaller than Cygnus A and ten times moredistant, consistent with the idea that the smallest radio sources might be path finders tofinding very distant galaxies. But a few months later Caltech radio astronomers identi-fied the first of several very small radio sources with what appeared to be galactic stars,thus again raising questions about the extragalactic nature of other small diameter radiosources.
2. The First Quasars
In late 1960, while searching for ever more distant radio galaxies, Caltech radio as-tronomers, John Bolton and Tom Matthews identified 3C 48 with a faint magnitude 16apparent stellar object. Apparently Bolton suggested that 3C 48 might not be a galacticstar but rather extragalactic with z=0.37. But, shortly later Bolton left Caltech to takeup his new responsibilities as the person in charge of completing the construction of thenew 210-ft radio telescope at Parkes, which completely occupied him for the next fewyears. At the 107th meting of the American Astronomical Society held in December,1960, Allan Sandage (1960) gave a late paper on ”The First True Radio Star.”In an exhaustive unpublished analysis of the complex 3C 48 emission line spectrum,Greenstein (1962) interpreted the observed emission lines in terms of highly ionized statesof various rare earth elements. He briefly speculated on a possible redshift of 0.37, butthen dismissed the possibility that 3C 48 was extragalactic. Nearly two years wouldpass, and two other compact radio sources, 3C 196 and 3C 286 were also identifiedas galactic stars. Based on OVRO interferometer measurements which had a nominal he Road to Quasars
3. Radio Quiet Quasars
The years following the recognition of the 3C 48 and 3C 273 redshifts led to theidentification of more quasars at ever larger redshifts and the recognition that quasarsare the extremely bright nuclei of active galaxies. Generally, the identified quasars had asignificant UV excess compared with stars, so due to the redshift of their spectrum, they K. I. Kellermannappeared blue on photographic plates facilitating their identification with radio sourceswith even modest position accuracy (Ryle & Sandage 1964).In 1965, Sandage noted that the areal density of blue stellar objects was some thousandtimes greater than that of 3C radio sources. Sandage argued that what he called quasistellar galaxies were related to quasars, except that they were “radio quiet.” Sandage’spaper was widely attacked, perhaps in part because it was received on May 15, 1965at the Astrophysical Journal, but S. Chandrasekar, the ApJ editor was apparently soimpressed by Sandage’s claim for a “New Constituent of Universe” that he held uppublication of the Journal, so Sandage’s paper would appear in the May 15 issue. TomKinman (1965) along with Lynds and Villere (1965) argued that most of Sandage’s BlueStellar Objects are just that, blue stellar objects, while Fritz Zwicky (1965) pointedout that he had previously called attention to this phenomena which he had called“compact galaxies” and he later accused Sandage of “one of the most outstanding featsof plagerism” (Zwicky & Zwicky 1971).As it turned out, most of Sandage’s quasi stellar galaxies were indeed galactic andthere are only some ten times more radio quiet quasars than radio loud quasars. But,it has now been more than half a century since we have divided quasars into the twoclasses of radio loud and radio quiet quasars, and it is still has not been clear if thereare two distinct populations or whether the radio loud population is merely the extremeend of a continuous distribution of radio luminosity. Proponents of each interpretationclaim that the other interpretation is due to selection effects. Many of the previousinvestigations designed to distinguish between radio loud and radio quiet quasars werelimited by contamination from low luminosity AGN with absolute optical magnitudesgreater than -23, by the use of samples based on radio rather than optical selectioncriteria, and by inadequate sensitivity to detect radio emission from most of the radioquiet population.In an attempt to overcome these limitations, Kimball et al. (2011) used the JanskyVery Large Array to observe 179 quasars selected from the SDSS. All of the quasars werewithin the redshift range 0.2 to 0.3 and were brighter than M i = -23 so were legitimatequasars. The radio observations were made at 6 GHs reaching an rms noise of ∼ µ Jy.All but about 6 quasars were detected as radio sources with an observed radio luminositysharply peaked between 10 and 10 Watts/Hz characteristic of the radio luminositytypically observed from star forming galaxies. About ten percent of the SDSS samplewere found to be strong radio sources with radio luminosities ranging up to 10 W/Hz.Kimball et al. concluded that the radio emission from radio quiet quasars is due to starformation in the host galaxy and is not directly related to the SMBH assumed to drivethe large optical luminosity of quasars.
4. Summary and Conclusions
Quasars and AGN are now part of the normal lexicon of astronomy. Although the1962 occultations of 3C 273 were crucial in bringing 3C 273 to the attention of MaartenSchmidt, the occultation position used by Schmidt for his Palomar observations wasin error by about 15 seconds of arc. A more accurate position had been establishedfrom OVRO interferometer observations but apparently was not accurately conveyed toSchmidt.Prior to the Parkes occulation, 3C 273 was not on anyone’s list of potentially inter-esting sources to be further studied, as it was already known not to be very small andthus presumably (and correctly) not very distant. Schmidt’s December 1962 spectro-scopic observations of 3C 273 were made only because he had received the occultation he Road to Quasars ∼
2. But, contrary to hopes, because they are not standard candles,quasars have had little impact to classical cosmology and the quest to define H or q .However, discovery of quasars has led to the recognition that they are driven by a centralengine, widely believed to be due to accretion onto a SMBH of ∼ or more solar masses.Over the half century since their discovery, quasars have had a profound impact tothe sociology of astronomy and astronomers as well as to astrophysics and cosmology.For the first decade or more, there was vocal minority that continued to advocate anon cosmological interpretation of quasar redshifts. The controversy was intense andsometimes very personal. The arguments never died, until their proponents died.
5. Unanswered Questions • Why did Greenstein, Matthews, Sandage and others wait two years to submit papersinterpreting 3C 48 as a galactic star. Why did John Bolton wait nearly thirty years beforegoing public with his claim to the discovery of the 3C 48 redshift? • Why did it take so long to identify 3C 273 with such a bright magnitude 13 opticalcounterpart, even though by the time of the 1962 Parkes occultation, at least three othersources, 3C 48, 3C 147, and 3C 196 had already been identified with apparent stellarcounterparts, and a Caltech interferometer position was apparently known to an accuracyof 5 or 10 arcseconds by the time of Schmidt’s May 1962 Palomar observations. • Why was the extragalactic nature of 3C 48 not accepted earlier as suggested by JohnBolton in November, 1960 and considered by Greenstein in his unpublished manuscriptsubmitted to the Astrophysical Journal? Why was a 3 or 4 Angstrom discrepancy amongthe derived rest wavelengths considered fatal to the z=0.37 interpretation when just afew months earlier Minkowski had gone out on a limb with the z=0.46 redshift basedon only one spectral line and a comparable radio luminosity? Why were Bolton andSchmidt ignored in the 1963 publications by Greenstein and Matthews (1963) and byMatthews & Sandage (1963)? • Why did it take Maarten Schmidt six weeks to decipher the simple Balmer spectrumof 3C 273? Was it possibly due to the intervention of holiday activities? Or, perhaps, itwas the receipt of Bolton’s January 26 letter, with improved radio positions, that inspiredSchmidt to re-inspect and finally understand the 3C 273 spectrum six weeks after he hadobserved it. K. I. Kellermann • Although all quasars presumably contain a SMBH which drives the excessive lumi-nosity, why are only about 10% of optically selected quasars also strong radio sources?
6. Acknowledgment
The National Radio Astronomy Observatory is operated by Associated UniversitiesInc. under Cooperative Agreement with the National Science Foundation. I am gratefulto many colleagues, especially Maarten Schmidt, Tom Matthews, Jesse Greenstein, AllanSandage, Cyril Hazard, and John Bolton who have shared with me their recollections ofthe events surrounding the discovery of quasars. Miller Goss and Ron Ekers brought the1960 and 1961 letters from John Bolton to Joe Pawsey to my attention. Dave Jaunceyprovided valuable comments, and John Faulkner alerted me to the 1981 recollection byHoyle of Bolton’s 1960 claim that 3C 48 was extragalactic with z=0.37.
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