Looking well beyond the Great Attractor.

Looking well beyond the Great Attractor 

Over and above the motion associated with the uniform expansion of the universe, our own galaxy and its neighbors have an additional motion. They seem part of a large-scale flow toward a distant, vast region of space dubbed the Great Attractor. The name suggests the presence of a large concentration of mass that gravitationally draws matter toward it. Now a team of Italian astronomers has identified another massive aggregation of galaxies in roughly the same direction but three times farther away.

The finding has important consequences for theories attempting to account for the distribution of matter in the universe. "Detailed study of tis region is very important, not only because no similar nearby concentration of $(galaxy$) clusters exists elsewhere, but also because of its direction," report R. Scaramella of the International School for Advanced Studies in Trieste, Italy, and his colleagues in the April 13 NATURE.

This newly discovered concentration of clusters packed with galaxies falls on a line nearly a billion light-years long that also runs through the Great Attractor, the local supercluster (which includes our own galaxy) and another large aggregation known as the Perseus-Pisces supercluster. Such a remarkable alignment on such a large scale is hard to interpret as mere coincidence, the researchers say.

"The issue is not whether these structures are lined up," says astronomer David Burstein of Arizona State University in Tempe. "They clearly are." What's more difficult to determine is whether the universe contains similar structures elsewhere. Burstein was a member of the international team of astronomers who first identified the large-scale galactic flow toward the Great Attractor (SN: 3/22/86, p.182).

"No definite conclusions can be drawn without a more complete picture of our surroundings," the Italian researchers say. Even the Great Attractor and the Perseus-Pisces region on opposite sides of the Milky Way are difficult to study, because dust surrounding and permeating our galaxy obscures the view in their directions. Nonetheless, the observation of such large features conflicts with theories suggesting the universe ought to be homogeneous on large scales.

Another question concerns whether the cluster containing our galaxy is actually moving toward the more distant aggregation. "It may be pulling us, but we don't see any evidence of that," says Burstein. Just as the moon, being closer though considerably less massive than the sun, exerts a greater tidal influence on the Earth, the Great Attractor's effect probably swamps any contribution from the more distant supercluster. There's also no evidence that objects in the Great Attractor region are themselves moving toward the supercluster.

Interpreting the observed motion of the galaxy clusters in complicated by the possibility that gravity may not be the only influence. On large scales, the structure of the universe itself comes into play. "To the degree that we can digest it, we have information here about conditions in the very early universe," says R. Brent Tully of the University of Hawaii in Honolulu. That leaves lots of room for theorists to try out their favorite cosmological models.