Radio waves may trace distant clustering.

Determining whether galaxies and quasars formed clusters early in the history of the universe can tell astronomers much about the distribution of matter and the evolution of structures in the cosmos. By focusing on distant and intense radio-wave sources, an astrophysicist has found evidence for large-scale clustering when the universe was less than half its current age.

Brilliant visible-light emissions from relatively nearby galaxies -- ones less than about 1 billion light-years from Earth -- permit astronomers to discern not only whether these celestial objects cluster, but also whether groups of these galaxies congregate in superclusters. In contrast, the faintness of more distant galaxies and quasars makes it all but impossible for astronomers to assess large-scale clustering far beyond the Milky Way -- at least when using visible light.

However, many distant galaxies and quasars do emit intense radio waves. Michael J. West at the Canadian Institute for Theoretical Astrophysics in Toronto, Ontario, took a cue from the behavior of nearby galactic clusters, called Abell clusters. About 20 percent of them contain a large, elliptical and bright galaxy at their center, called a "cD" galaxy, he notes. Other researchers had previously established that a cD galaxy tends to emit visible light pointing toward another cD galaxy -- if the neighboring galaxy resides within 150 million light-years. Astronomers had also found that the parent Abell cluster housing the cD galaxy exhibits a similar behavior -- it tends to emit light toward a neighboring cluster located no more than 150 million light-years away.

Notes West: "It is certainly remarkable that the structure and orientation of cD galaxies, just a few thousand light-years in diameter, should somehow reflect the presence of large-scale clustering of matter on scales of millions of light-years."

He wondered if the orientation of radio-emissions from distant galaxies could play a role similar to that of visible-light emissions from cD galaxies. If the analogy held, then such distant radio emissions could literally point to superclustering that occurred when the universe was much younger.

To more completely examine his hypothesis, West reviewed radio-wave surveys of nearby cD galaxies. And "sure enough, in almost every case," he says, "the radio emission from the [nearby] cD galaxy was indeed strongly aligned with [visible-light emissions] and the cluster in which it was located."

Other researchers had investigated whether radio emissions from distant galaxies aligned with the direction of the fainter, visible-light radiation from these objects. A wide variety of surveys indicated that distant galaxies -- those 5 billion to 16 billion light-years away -- indeed emitted both types of radiation along roughly the same direction.

Then came the finding that helped clinch his argument, West says. An analysis of 600 distant galaxies revealed that 38 had neighboring quasars located within 150 million light-years. Radio emissions from all 38 pointed toward the quasars. West observes that such alignment appears highly similar to the clustering that takes place among nearer cD galaxies and Abell clusters.

"It's a leap of faith," admits West, to extrapolate from what's known about nearby galactic clusters to more distant ones "where all we can see is the radio emissions." But, he adds, "The most natural interpretation of these results would seem to be that they indicate a well-organized pattern of superclustering [among distant galactic groups] when the universe was less than half its present age."