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Cygnus A - Part II
Cygnus A, Quasars, and Quandaries

March 13, 2001 ::

Synchrotron Radiation: Electrons moving in magnetic field radiate photons.
(CXC)

When Walter Baade and Rudolph Minkowski obtained the spectrum of Cygnus A, they found what they considered to be proof of colliding galaxies: emission lines that indicated they were produced by gas in a high state of excitation. This evidence, combined with the facts that Cygnus A had a distorted shape and was located in the center of a galaxy cluster where collisions of galaxies were more probable, seemed to clinch the case.

But there were some major problems with this interpretation. The spectrum also showed that the emission lines were all red-shifted by the same amount. This in itself was not unexpected, since two decades earlier Edwin Hubble and his colleagues had shown that the spectra of distant galaxies would show a red-shift proportional to their distance. The difficulty was that the measured red-shift implied that Cygnus A was very distant -- possibly 1 billion light years distant!

Radio image of Cygnus A
This Very Large Array image of Cygnus A is characterized by very faint, narrow jets, distinct lobes, and hot spots at the ends of the jets.
(NRAO/AUI)

At such a large distance Cygnus A would need an extremely powerful energy source to produce the observed radio and optical radiation. It seemed impossible that galaxies in collision could generate such intense power. This problem was made even worse in the mid-1950's by additional radio observations and theoretical work. The conclusion from these efforts was that radio waves from Cygnus A and other radio-emitting galaxies are produced by high-energy electrons spiraling around magnetic field lines. This process, which was first observed in a particle accelerator called a synchrotron, is called synchrotron radiation.

At a meeting of radio astronomers in Paris in the summer of 1958, Geoffrey Burbidge discussed the implications of synchrotron radiation coming from Cygnus A. He showed that the energy needed to produce the high-energy particles was much greater than the expected energy from a collision of galaxies.

Still more problems for the collision theory became apparent when R.C. Jennison reported on new measurements of the structure of the Cygnus A radio source. It has the shape of a great cosmic dumbbell, with two huge lobes of high-energy particles located over half a million light years apart. The galaxy, which is several times smaller than the lobes, is located in the middle. It was beginning to look as if the radiation from Cygnus A was not due to galaxies in collision, but to some mysterious explosive process where high-energy particles were being blown out of the galaxy!

Sir Fred Hoyle, 1972
(Courtesy of The Archives, California Institute of Technology.)

The search for a way to produce high-energy particles soon took an unexpected turn. Fred Hoyle pointed out that if other powerful radio galaxies had sources as large as Cygnus A, they could be seen at distances much greater than that of Cygnus A. If the sources were all about the same intrinsic size, then a measurement of their apparent size could give astronomers vital information about the nature of the universe. For example, the Steady State universe championed by Hoyle, Thomas Gold and Hermann Bondi would show a steady decrease in angular size, whereas the Big Bang universe would show a decrease only up to a point before it started to increase because of the curvature of space.

This ingenious insight spurred a flurry of activity and intense work by radio astronomers to determine the size of faint radio sources, and settle the issue of whether the universe was in a steady state, or originated in a Big Bang. They found many new radio galaxies and scientists on various sides of the issue soon became embroiled in a bitter debate as to the significance of the results.

In the meantime, radio astronomers found that some of these newly discovered radio sources were difficult or impossible to resolve optically, even with their largest and most sensitive telescopes. Thomas Matthews & Allan Sandage used the Palomar 200 inch telescope to identify one of them, called 3C48, with a faint blue, starlike object. The object presented a profound puzzle. It had the colors of a white dwarf, but a wisp of nebulosity that had never been seen in association with a white dwarf. Most mystifying of all was the spectrum. Like Cygnus A, it contained emission lines, indicating gas at ten thousand or more degrees in the vicinity of the object. Unlike Cygnus, or any other object they had ever encountered, they could not identify the emission lines with any known elements!

Optical Spectrum of 3C273
(AAS/ADS)

Over the next two years, many spectra were taken of 3C48 by a number of different astronomers, but no one could identify the emission lines. Then, in 1963, the Australian radio astronomers Cyril Hazard, M.B. Mackey and A.J. Shimmins achieved a breakthrough by using the fact that the moon was eclipsing another radio source, 3C273.

Since the position of the edge of the moon is known very accurately for any time, by carefully timing the disappearance and reappearance of the source, they could measure the position and size much more accurately than previously possible. They found that 3C273 is a double radio source, with one component lying on top of blue starlike object.

Optical Spectrum of 3C 48
(AAS/ADS)

With this accurate position, Maarten Schmidt of Caltech used the Palomar 200 inch telescope to photograph 3C273, and take a spectrum. The photograph showed a starlike object with a faint wisp or jet off to one side. The spectrum showed mysterious, broad emission lines like those seen in the spectrum of 3C48.

Maarten Schmidt, 1965
(Courtesy of The Archives, California Institute of Technology.)

Then Schmidt had one of those simple, yet profound insights that one wonders why it was missed by so many others. He showed that the mysterious lines were due to well-known transitions between energy states in a hydrogen atom. They were merely shifted to the longer wavelengths (to the red end of the spectrum) by about 16 percent.

This was not the largest red-shift observed to that point -- Minkowski had shown that the galaxy 3C295 and its associated galaxy cluster had a red-shift of 46 percent. The problem, as expressed by Schmidt in his 1963 paper describing his discovery, was "the unprecedented identification of the spectrum of an apparently stellar object in terms of a large red-shift... " How could a starlike object have such a large red-shift?

Schmidt offered two explanations: 3C273 was (1) a stellar object in which the red-shifts were caused by a strong gravitational field -- this was difficult to reconcile with the details of the spectrum; or (2) the nucleus of a galaxy that was moving away from us at a velocity of 170 million kilometers per hour.

Optical image of 3C273
(AAS/ADS)

Using the Hubble red-shift-distance relation, the latter interpretation implies that 3C273 is more than 2 billion light years away! This in turn means that the nucleus of this galaxy is about 100 times brighter than the luminosity of an entire bright galaxy. Yale University astronomers Harlan Smith and Dorrit Hoffleit used Harvard Observatory's historical photographic plate collection to show that 3C273 had been varying its light output significantly over the period of a year.

Soon after Schmidt's discovery, the spectrum of 3C48 was reexamined by Jesse Greenstein and Matthews at Caltech. Unlike 3C273, the hydrogen lines were not apparent, which explains why they may have missed the red-shift interpretation. Nevertheless, by using the red-shift hypothesis, they were able to fit the spectrum to lines from the elements magnesium, neon and oxygen, all red-shifted by 37 percent.

These discoveries happened so quickly that they were reported in the same issue (March 16, 1963) of the journal Nature. This date marks astronomers' discovery of one of the most important constituents of the universe -- quasi-stellar radio sources, a.k.a. quasars, a.k.a. quasi-stellar objects, a.k.a. QSOs.

Was a quasar a totally different object, or was it an extreme example of an explosive galaxy like Cygnus A? What could be the source of the immense power in quasars and in the centers of radio galaxies? These questions were on the minds of astronomers and astrophysicists as they gathered in Dallas, Texas in December of 1963 at a symposium on "Quasi-stellar Sources and Gravitational Collapse."

A smorgasbord of ideas was advanced for the energy supply of quasars: matter-antimatter annihilation, chain reaction of supernovas, the collapse of a cluster of stars, and the collapse of a superstar millions of times more massive than the Sun. Most of the talk turned to the collapse of something, because, as Philip Morrison of MIT said, "the evidence strongly favors the look of a gravitational event."

The supermassive star model attracted the most attention, partly because it had been worked out in more detail, and partly because its authors were Hoyle and his colleague William Fowler of Caltech, two of the most innovative and respected astrophysicists of the day. However, their reputation did not immunize them from criticism. Freeman Dyson of the Institute of Advanced Study put his finger on a major problem: once the supermassive star's collapse starts, it will be over in a day! Ways out of this dilemma, such as rapid rotation, or fragmentation were discussed, and there was mention of the ultimate fate of the matter, of gravitational event horizons and similar bizarre ideas that would eventually come to occupy center stage. These ideas involved black holes, but they were not mentioned, because the term "black hole" would not be coined until 1967.

In the meantime, the consensus was that Minkowski had been right to drink Baade's bottle of whiskey. Cygnus A most likely was not due to the collision of galaxies.

Next Time: Astronomers are drawn to black hole power!

References:

I. Robinson et al. Quasi-Stellar Sources and Gravitational Collapse Chicago: U. Chicago Press, 1965.

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Cygnus A - Part IICygnus A, Quasars, and Quandaries

Cygnus A - Part II
Cygnus A, Quasars, and Quandaries