Copyright © 1999 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 1999. 37: 445-486
Copyright © 1999 by Annual Reviews. All rights reserved |
7.1. Initial Identification of Radio Sources
What would develop into the present major discipline of relativistic astrophysics began with the "rediscovery" of cosmic radio waves. The initial discovery of Jansky (1932, 1933, 1937, 1939), and the genius and tenacity of Reber (1940a, b, 1942, 1944) in advancing the discovery are too well known to again recount here.
However, the major advances came after World War II when radio telescopes were first brought into operation. For the history of this early period we can refer the reader to articles and books by Hey (1946, 1973), Hey, Parsons, & Phillips (1946), Moffett (1975), Sullivan (1982, 1984).
The connection with Walter Baade and Rudolph Minkowski at Palomar came after several groups of radio astronomers began to detect discrete sources of radio emission that needed to be identified with optical objects in the sky. Interferometric measurements in Australia and at Cambridge, England led to the discovery of several strong sources. The first definitive optical identifications were made by Baade and Minkowski (1954a, 1954b) with the 200-inch telescope, based on radio data provided by Bolton and Stanley (1948) who found the diameter of the Cygnus source to be less than eight arcmin; Bolton (1948) who added to the list of discrete sources; Ryle & Smith (1948) with a good interferometric position of Cas A; Bolton, Stanley, & Slee (1949) who had an early good radio position of Virgo A; and Smith (1951) who provided a very accurate radio position that coincided with the Crab nebula.
The three principal competing radio astronomy groups were strong rivals, each attempting to play a dominant role in the highly charged identification game, but each trusted Baade and Minkowski to play fairly. These optical astronomers kept all the radio-source-position information, communicated to them privately, strictly compartmentalized. It is clear from the content of the 1954 papers by Baade & Minkowski that each radio group had sent their new radio positions to Pasadena before publication, permitting the optical identifications to be made using the Palomar 48-inch and 200-inch telescopes.
In the early 1950s Caltech began a radio astronomy program, and John Bolton, an English physicist who was leading a radio astronomy group at CSIRO in Australia, was invited to develop a Caltech radio observatory. Starting from nothing in 1954, he built the Owens Valley Radio Observatory.
But before choosing that radio-quiet site, Bolton mounted his first antenna on the Palomar grounds, some 500 yards west of the 200-inch dome. He had recruited from Australia Gordon Stanley and J.A. Roberts, and also attracted graduate students and postdoctoral fellows to begin a dominant research program at Palomar before the completion of the permanent site in Owens Valley.
At the beginning, Bolton and his associates were still radio physicists, not astronomers. This led to wonderful accounts of their early encounters with the astronomy of the celestial sphere. The same problems were encountered by Ryle and his colleagues in Cambridge. Baade told of times when he and Minkowski began receiving radio positions from England and Australia from the several radio physics groups. Minkowski would write back asking for the equinox used in reporting the positions, to which questions were asked back as to "what do you mean by the equinox?" Not only were the astronomers rapidly being educated in the new world of radio physics, but the radio scientists soon learned about the celestial sphere. (4)
What was the connection of this new radio astronomy with relativistic astrophysics and, as it turned out, again with Palomar? The connection came through the fact that the bulk of the radio emission is incoherent synchrotron radiation emitted by high energy electrons (typical energies of approximately 1 Gev), spiraling in weak magnetic fields (approximately 10-3 to 10-5 gauss). The developments that led to that conclusion are well summarized by Ginzburg & Syrovatskii (1964).
If synchrotron emission is the predominant process, it requires that (1) relativistic electrons and magnetic fields be present in the objects that are parents to the radio sources, and (2) that parts of the radiation must be polarized. Concerning item (1), Ginzburg (1953) had shown that relativistic electrons must be formed continuously in the interstellar gas of the galaxy by collisions of relativistic protons, already known in the cosmic rays reaching the earth, on interstellar atoms. Furthermore, in a very important paper, Pikelner (1953) had emphasized that magnetic fields exist in the interstellar medium throughout the galactic system. Concerning item (2), polarization of the galactic radio noise was believed to have been measured by the Soviet radio astronomer Razin (1956, 1957).
The Russians had also predicted that if this theory was correct, it might be possible to detect linear polarization of the optical light which in a few cases might be emitted by the synchrotron process. They first applied the argument to the radiation from the Crab. Optical polarization was first discovered in the Crab Nebula (Vashakidze 1954, Dombrovsky 1954). Oort & Walraven (1956) then published what has become a classic paper on observations and theory of the Crab radiation, settling the question of the physical process. Subsequently, Baade (1956c) published exquisite photographs made with the 200-inch that dramatically showed the polarization. These photographs were analyzed by Woltjer (1957). The result showed that the Crab Nebula contains a significant magnetic field and a reservoir of relativistic electrons.
A second prediction was made by Shklovsky (1955) that the jet in the center of M87 (the radio source Virgo A) would show optical polarization demonstrating that the synchrotron mechanism was ubiquitous in galaxies as well as in galactic objects such as the Crab. Acting on Shklovsky's suggestion, Baade (1956b), using the 200-inch, discovered polarization in the jet of that galaxy using a polarizing filter and photographic plates.
The importance of the discovery and an analysis not only of the energetics but also of the origin of the relativistic electrons were set forth in an important paper by Burbidge (1956b). This was the second of a number of papers on the energetics of radio sources by Burbidge (1956a, 1958, 1959) who was by then a Carnegie Fellow of the Mount Wilson and Palomar Observatories. Burbidge was the first theoretician appointee to the Observatories fellowship program, beginning a long and distinguished career in activities connected in many ways with the Observatories and with Caltech.
4 A story as remarkable as it is true, verified by those who worked regularly on Palomar mountain, is the day when Bolton aligned the polar axis of his equatorially mounted, temporary, 25-foot Palomar dish. The antenna could be adjusted in azimuth once the direction of true north was established. The alignment was accomplished one sunny day when Bolton pounded a stake in the ground near the antenna and watched the direction of the shadow as the sun rose toward noon. When his watch, set to Pacific standard time, read 12 noon, Bolton marked the line of the shadow and made his polar axis parallel to it, clamping the antenna azimuth adjustments home. It was soon thereafter that Bolton decided he should have a few astronomy postdoctoral astronomical fellows join the project. One of these was T.A. Matthews, a recent Harvard Ph.D., who later played such an important role in the discovery of quasars. Back.