Chandra :: Chronicles :: The Attraction of Black Holes

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Cygnus A - Part III (Conclusion)
The Attraction of Black Holes

April 17, 2001 ::

Chandra X-ray image of Cygnus A
(NASA/UMD/A.Wilson et al.)

In the 1960s the search for more radio galaxies like Cygnus A led to the discovery of quasars. These mysterious cosmic sources generated a hundred times more power than a large galaxy of several hundred billion stars, all from a region much smaller than a normal galaxy.

This threw astrophysicists into a quandary: how could so much energy be produced in such a small region? Some astronomers thought that the problem was so great that quasars couldn't possibly be as far away as their redshift indicated, that they might be objects ejected from a nearby galaxy, or perhaps they represented new physics.

Schematic of a Jet
(D. Meier et al, Science)

Another suggestion, made by Edward Teller, was that the prodigious energy source of quasars was due to the collision of huge clouds containing 10 million solar masses of matter and antimatter. Small bits of antimatter can be produced by collisions between particles with high-energy, but there is no evidence for such large concentrations of antimatter in the universe. Furthermore, the primary result of such collisions is gamma rays, not the combination of high energy jets, magnetic fields, and radio, optical, ultraviolet, X- and gamma radiation seen in quasars.

Nuclear power also seemed an unlikely source. Although that is what powers stars, it would be impossible to pack a hundred billion Suns into a region one light year in diameter. Stars more massive than the sun radiate more efficiently, but such a dense collection of stars would collide and explode as supernovas, or merge to form a supermassive star, which would then implode. Ultimately, scientists concluded, the source of energy for quasars had to be gravity, the familiar force that had been known since Isaac Newton to operate throughout the universe.

But this was not Newton's brand of gravity -- it was Einstein's, in which gravity is described not as a force, but as a warping of space due to the presence of matter, and energy is the equivalent of matter. So the more energy you pack into a small space, the more mass is concentrated there, the greater is the warp in space and the stronger the concentration of energy, leading to a vicious cycle of unending collapse to . . . to what?

Schematic of the black hole accretion disk model for Cygnus A
(CXC/K.Kowal)

In the 1960s, research on the ultimate fate of massive stars was moving on a parallel track with the work on quasars and radio galaxies. An increasing number of physicists were coming around to the point of view that the collapse of the core of a sufficiently massive star would produce what John Wheeler called, a "black hole." Except for the gravitational warp, which all matter in its vicinity would feel, a black hole is closed off from the universe. An area called an event horizon is formed around the black hole. Matter, or light that has passed beyond the event horizon has a one way ticket to oblivion. The only evidence of its prior existence is that the mass of the black hole has increased, and its rotational speed has changed.

Edwin Salpeter
(Photo 1987, courtesy Cornell University )

In 1964 Edwin Salpeter at Cornell University and Yakov Zeldovich of the Institute of Applied Mathematics in Moscow independently proposed that a stream of gas falling toward a black hole could in principle be heated to very high temperatures, where it would produce X-rays. Given enough matter, the X-rays would be detectable. Zeldovich and his Russian colleague, Igor Novikov, proposed that a black hole that is part of a double star system would have the best chance of being detected.

Yakov Zeldovich
(Photo ca. 1950, courtesy Prof. V. I. Goldanskii)

The confirmation of this idea came in 1971 when combined X-ray, optical and radio observations led to the conclusion that the X-ray source Cygnus X-1 is very probably due to the infall, or accretion of matter from a bright blue star onto a black hole with the mass of about 10 solar masses.

Donald Lynden-Bell
(Photo courtesy Prof. Lynden-Bell)

This discovery opened the way for the eventual acceptance of a black hole model as the central powerhouse of quasars. Donald Lynden-Bell and Martin Rees of Cambridge University in England showed that a gigantic black hole in the center of a galaxy could produce the necessary energy if it swallowed matter, or gas, in an amount equal to about one Sun per year. The gas would form a disk, called an accretion disk, around the black hole. Because of friction between the gas particles in the disk, they would gradually slowly lose energy and move inward or in the words of Lynden-Bell and Rees, "slowly run down into the central black hole just as water runs out of a bath." The glow from the outer portion of the heated disk would produce the optical radiation seen in quasars. X-radiation would be produced in the extremely hot inner parts of the disk.

Roger Blandford
(Photo courtesy of Caltech)

In subsequent work, Lynden-Bell, Rees and their colleague Roger Blandford, who is now at Caltech, showed that the swirling motion of the gas in the quasar accretion disk would strengthen any magnetic field present, creating two tornados of spinning magnetic field lines on the top and bottom of the disk. These whirling magnetic fields could then generate the powerful jets of high-energy particles and magnetic fields that are seen to blast out from the centers of radio galaxies.

As telescopes became more sensitive and better able to probe the inner parts of radio galaxies and study quasars in more detail, evidence in favor of the black hole model has grown. Radio, infrared and optical telescopes have detected matter swirling around dark, massive objects in the centers of more than two dozen galaxies, and bright central pinpoints of X-radiation have been observed in most of these objects. Radio and X-ray jets have been traced back to the nucleus as well.

Martin Rees
(Photo courtesy Prof. Rees)

In discussing the black hole accretion disk model, Rees commented that "It is a kind of rough caricature, though we may hope that, like a good caricature, it highlights rather than obscures the essence of the phenomenon."br />
There are many puzzling details that remain to be explained, such as how the supermassive black hole formed, and how the high-energy jets can maintain their spike-like shape over hundreds of thousands of light years. The resolution of these mysteries may lead to the discovery of entirely new phenomena, just as the first explorations of the nature of the radio waves from Cygnus A led to the discovery of quasars and supermassive black holes.

As an epilogue to the bet between Baade and Minkowski, the question of whether Cygnus A is due to a collision of galaxies is still unresolved. The unusual shape of the galaxy is due to dust and gas clouds, but it may harbor a double, or even triple nucleus. The energy source is most likely caused by the accretion of gas into a gigantic black hole, but the origin of this gas may have been a collision with another galaxy! If they were still around, they would probably have to split a bottle of whiskey and call it a draw.

References:

K. Thorne, Black Holes and Time Warps (New York: W.W. Norton, 1994)
M. Rees, Before the Beginning (Reading, MA: Addison-Wesley, 1997)
D. Meier et al. Science 291, 84 (2001)
C. Carilli and P. Barthel, Astronomy & Astrophysics Reviews, 7, 1 (1996)

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Cygnus A - Part III (Conclusion)The Attraction of Black Holes

Cygnus A - Part III (Conclusion)
The Attraction of Black Holes