Distant cluster suggests low-weight cosmos.

Lured by the radio beacon of a faraway galaxy, astronomers have discovered the most distant cluster of galaxies known in the universe. The group lies 10 billion light-years from Earth and provides a snapshot of what a large collection of galaxies looked like when the cosmos was still in its youth.

The distant cluster also provides new evidence that the density of matter is lower than the so-called critical density, the value required to keep the universe flat. Above or below that density, the universe has to have a curved geometry, notes study coauthor Andrew C. Fabian of the University of Cambridge in England. He and his Cambridge colleagues Carolin S. Crawford, Stefano Ettori, and Jeremy S. Sanders will report their findings in an upcoming MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY.

To search for distant clusters, the team scoured the heavens for a signpost: The gas surrounding large groups of galaxies emits a diffuse, X-ray glow. Even so, says Fabian, the team would have had to examine thousands of images taken by NASAs Chandra X-ray Observatory to find such a signpost. So the researchers sought out another indicator. In nearby reaches of the universe, galaxies that emit powerful radio waves tend to reside in clusters. The team predicted the same would hold for powerful radio emitters in more distant parts of the cosmos.

Aiming Chandra at the radio-loud galaxy 3C294, 10 billion light-years from Earth, the astronomers struck gold. They found the signature X-ray glow indicating the distant body was part of a cluster. This radiation left the galaxy cluster 10 billion years ago, revealing the character of the cluster when the universe was just 20 percent of its current age.

"Our cluster isn't equivalent to a living dinosaur, but it is the most massive distant, gravitationally bound object known," Fabian says.

Finding a cluster that hails from such an early time supports the idea that the cosmos has a low density of matter, the team notes. In a low-density cosmos, large structures such as galaxy clusters had to have formed early, when the expanding universe was still relatively compact. Later, the density of matter would be too low for gravity to pull galaxies together.

Many studies have indicated the universe is seriously underweight, its distribution of matter far below the critical density. Even so, observations of the cosmic microwave background, the hiss of radiation left over from the Big Bang, suggest the cosmos is, in fact, endowed with the critical density required to make it flat (SN: 4/29/00, p. 276).

Since there doesn't seem to be enough matter to do the job, cosmologists have proposed that some sort of mystery energy, dubbed dark energy, makes up the density deficit. Dark energy has a startling property--it accelerates cosmic expansion (SN: 2/12/00, p. 106).

If the cosmos is both underweight and contains dark energy, then the newfound cluster is just one of hundreds awaiting discovery, notes August E. Evrard of the University of Michigan in Ann Arbor.