Images hint at seeds of a giant galaxy.

This could be the start of something big.

Peering far back in cosmic time, the Hubble Space Telescope has spied a group of 18 diminutive youngsters that may represent the building blocks of a giant galaxy like the Milky Way. Huddled together, these starry bodies appear poised to merge and, over time, to form one or two large galaxies, says Rogier A.

Windhorst of Arizona State University in Tempe.

Indeed, four of the objects have double centers, indicating that they have already coalesced with a neighbor.

If Windhorst and his colleagues are right, the Hubble image has captured the pieces of a galaxy before they had assembled, providing a striking illustration of the bottom-up theory of galaxy formation, in which large galaxies arise from the merger of smaller objects. Although astronomers have found many tiny, compact galaxies still in their first blush of youth (SN: 2/24/96, p. 120), Windhorst notes that no one has ever seen so many of these objects bunched together in a patch of sky as small as 2 million light-years across.

The Hubble observations suggest, but don't prove, that the 18 objects, each about one-fiftieth the diameter of the Milky Way, all lie at the same distance from Earth. Ten of them certainly do: Observations with ground- based telescopes place them about 11 billion light-years away. This indicates that Hubble has provided a snapshot of how these objects looked when the universe was 11 billion years younger, or 10 to 20 percent of its current age.

If these youngsters are seen as they were about to assemble into a single large galaxy, the images would also uphold the cold dark matter theory of galaxy formation. This theory, which is consistent with the bottom-up theory, holds that invisible, slow-moving particles make up most of the universe and that giant galaxies can't form earlier than a billion or so years after the birth of the universe. That timing dovetails with Hubble's observation of these 18 bodies, Windhorst and his collaborators assert in the Sept. 5 Nature.

Another interpretation of the findings refutes the merger notion and might sound the death knell for cold dark matter in its standard form.

Several astronomers, including Judith G. Cohen of the California Institute of Technology in Pasadena and Mark Dickinson of the Space Telescope Science Institute in Baltimore, suggest that the objects discovered by Windhorst's team aren't fragments of a giant galaxy-to-be but bona fide galaxies in their own right. The grouping would then constitute a young cluster whose galaxies had formed much too early for the cold dark matter model to explain.

Cohen notes that without further data, it's impossible to determine whether the 18 objects were about to coalesce or just remain clustered. She notes that researchers have found hints of galaxy clustering even further back in time and that her own studies show clustering at a time when the universe was about half its present age (SN: 6/29/96, p. 406). Cohen adds that if the objects do serve as building blocks, then the cosmos must have contained many more galactic objects in the distant past than it does today, a finding that may not square with the counts of galaxies at different epochs of the cosmos.

Windhorst says that if surveys of adjacent areas of sky were to add more objects to his grouping, then he, too, would favor the clustering hypothesis.

He comments that if the 18 objects, which glow with the blue light of young stars, are common in other parts of the sky, they could account for the excess of faint blue objects seen at great distances. - R. Cowen