US astronomers peer through galactic lens to find cosmic mirage
A transcontinental team of US astronomers has found a cosmic mirage. It is the double image of a distant object formed by the gravitational field of a massive galaxy. The galaxy is acting as a giant lens.
Just as hot air over a sun-drenched desert bends light near the ground, so the galaxy's gravity bends light (or radio waves) passing close by. In either case, multiple images of a distant object will be formed - that is, a mirage.
Cosmic mirages of this type have long been predicted as a consequence of Einstein's General Theory of Relativity. But none was observed until 1979. This latest find is only the fourth to be reported.
Nevertheless, as the discoverers point out, astrophysicists already consider the gravitational lens an important new tool for cosmic study. It can help estimate the distribution of mass in galaxies and clusters of galaxies. It can aid in evaluating such basic factors as the rate of expansion of the universe.
The new lens system was found through the collaboration of three research teams. They include Charles E. Lawrence of both the Massachusetts Institute of Technology (MIT) and the California Institute of Technology (Caltech); Donald P. Schneider and Maarten Schmidt of Caltech; Charles L. Bennett, Jacqueline N. Hewitt, and Bernard F. Burke of MIT; and Edwin L. Turner and James E. Gunn of Princeton University. Technical details of their research, which was announced simultaneously by the three universities, will be published in the Jan. 6 issue of Science magazine.
As with the early lens discoveries, these astronomers are looking at images of a distant quasar (quasi-stellar object). Quasars are very energetic, compact entities. They pack the power of a whole galaxy of stars in less volume than the core of a normal galaxy. They also are among the most distant objects in the universe. The quasar in question is near their outer range.
Astronomers measure cosmic distances in terms of what they call the red shift. The faster an object moves away from us, the redder this recession makes its light appear. Also the farther away an object is, the faster it is receding as part of the overall expansion of the universe. Thus, the more distant an object, the larger its red shift.
This quasar has a red shift of 3.27 - one of the largest on record. To estimate the distance this represents, astronomers have to make assumptions about the large-scale structure of the universe, especially about how fast the velocity of recession increases with distance. Using one of several standard conversion equations, a red shift of 3.27 corresponds roughly to a distance of 9 .23 billion light years.
The galaxy which acts as the lens is much closer. It has a red shift of only 0.8, corresponding to a distance of 6.75 billion light years.
Commenting on this, Charles Lawrence points out that the quasar is so distant and faint that it probably would be difficult to detect were it not for the enhanced brightness due to the gravitational lens. He says that this, in effect, adds to the power of the Earth-based equipment that located the phenomenon.
The discovery is the first fruit of a complex search program which is expected to turn up more cosmic mirages. First, radio telescopes are used to search for possible gravitational lenses among thousands of radio sources. When a likely candidate is found, as in this case, a detailed study is made with the Palomar Observatory's 200-inch telescope.
With the aid of a very sensitive electronic light sensor called a charged coupled device, this famous telescope is still one of the most powerful in the world for deep space observing. Dr. Lawrence says, ''It still is at the frontier of research. Working with it, I found it didn't feel like an antique at all.''
While the lensing effect interests astronomers in its own right, Lawrence explains that it is even more valued for the new information it can yield about the large-scale nature of the universe. As more of the lenses are found, he says , astronomers expect to be able to study the way mass is distributed in galaxies. This is because the lens effect depends on this mass distribution. Such study will give astronomers a better appreciation of the overall mass distribution in the universe. Also, the details of how the images are formed will help in estimating such fundamental characteristics of the cosmos as whether or not the universe's expansion is slowing down.
It is the hope of this kind of scientific payoff that has encouraged astronomers to consider the discovery of gravitational lenses one of the most important developments in their science in recent years.