The Mysterious Celestial Twins

A pair of distant quasars stirs astronomers

Of all the objects that astronomers have found in the skies, quasars are the most enigmatic. Though they resemble ordinary stars when viewed through optical telescopes, they are billions of lightyears* away, the farthermost objects in the observable universe. But to be seen through ordinary telescopes and detected by radio telescopes at those distances, quasars must radiate more energy than entire galaxies, which are giant islands of billions of stars. Now, while trying to explain what quasars are and how they radiate so much energy, astronomers have been confronted by yet another mystery: twin quasars.

The first paired quasars were discovered last March by a trio of astronomers, Britons Dennis Walsh and Robert Carswell and American Ray Weymann, using the 2.1-meter (82-in.) telescope at the Kitt Peak National Observatory in Arizona.

The quasars were about 10 billion light-years from earth (meaning that the light detected at Kitt Peak had left the objects 10 billion years ago), and both were receding from the earth at two-thirds the speed of light. What was most unusual was that they were only some 150,000 light-years apart--a stone's throw by cosmic standards--and had virtually identical light spectrums, which meant that their physical characteristics, as well as their velocities, were the same.

To the astronomers, this was too much of a coincidence. Writing in Nature last May, they suggested that what they might be seeing was two images of the same quasar. How was this possible? More than half a century ago, scientists realized a bizarre consequence of Einstein's general relativity theory: if a very massive object were located almost directly between the earth and a distant star, its tremendous gravity would act as a "gravitational lens" that could bend the starlight into two different paths. To produce the effect observed at Kitt Peak, the astronomers calculated, a huge galaxy or a black hole at least 10 trillion times as massive as the sun would be required.

The Nature report created a wave of excitement among scientists, and several teams focused their attention on the twin quasars. Among them were David Roberts, Perry Greenfield and Bernard Burke, all from M.I.T., who analyzed signals from the quasars received at the Very Large Array (VLA) antennas of the National Radio Astronomy Observatory near Socorro, N.M. What resulted was a radio map that, with one important exception, coincided with the images seen with the Kitt Peak telescope. The difference was that the sensitive radio antenna array discerned two jets of material that seemed to be shooting from one of the quasars. Explains Physicist Burke: "Quasars do have outbursts and send out material that gives off radio noise without producing much light."

That would explain the difference between the photograph and the radio picture. Trouble is that if the quasars are really twin images formed by a gravitational lens from the light of a single quasar, they should be mirror images of each other. But, as Burke points out, "we don't see mirror images of the spraying material with the other quasar." The M.I.T. team and others plan more radiotelescope observations in the hopes of confirming that the jets are being ejected from one quasar and detecting similar ones streaming from the other. Concludes Burke: "If the mirror images are not found, it will make life very difficult for those who propose a gravitational lens."

Then how do the M.I.T. scientists explain the remarkable similarity in the velocities and chemical characteristics of the quasars? If both formed and evolved at about the same time and in the same environment, they say, there is every reason for them to be virtual twins. If they are indeed twins, scientists will try to make use of their proximity. Explains Burke: "If this is a pair of quasars and we find evidence of interaction, we could then measure their masses and get a better handle on what the nature of these objects might be." And because scientists see the quasars now as they were 10 billion years ago, studying the twins should teach them more about what the universe was like when it was very young.

* A light-year is 5.8 trillion miles, the distance that light travels in a year.

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