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Using the Hubble Space Telescope to peer back in time, astronomers have found the first direct evidence that spiral galaxies may have been more common in clusters of galaxies several billion years ago than they are today. One of the Hubble pictures also reveals a smaller galaxy cluster that might be the most distant grouping of galaxies ever imaged.

These findings suggest that some spirals that existed in the early universe are the ancestors of galaxies in today's clusters, says Alan Dressler of the Observatories of the Carnegie Institution of Washington in Pasadena, Calif.

Dressler and his colleagues, including Augustus Oemler of Yale University, began their study as a follow-up to ground-based observations that distant clusters of galaxies have an unexpectedly blue appearance. While telescopes on the ground couldn't discern the shapes of individual galaxies in these faraway clusters, the blue color is associated with spirals, the galaxies most likely to be in the throes of starbirth. In contrast, nearby clusters -- known to contain many elliptical galaxies -- appear redder, indicating very little star formation.

Despite its flawed optics, Hubble has now revealed the shapes of the galaxies -- an atlas of youthful elliptical, spiral, and lens-shaped bodies -- in a pair of distant clusters. Each cluster lies about 4 billion light-years from Earth, meaning that Hubble views the clusters as they appeared 4 billion years ago. At a press briefing this week in Washington, D.C., Dressler reported that 30 percent of the galaxies in these clusters are spirals, compared with just 5 percent in more modern, nearby clusters.

So where did all the spirals go? Dressler notes that they might not have disappeared, but in their old age may simply have stopped forming so many stars and thus faded from view. But the Hubble pictures show that the churning, "Cuisinart" environment of dense clusters can rip spirals apart and cause colliding spirals to merge. And such mergers might transform the flat disks of spiral galaxies into the round balls of ellipticals, he says.

To determine whether spirals really are the ancestors of ellipticals, researchers need to examine groupings of galaxies even more distant, Dressler notes. One of the Hubble images may contain such a grouping. An enlargement of the image reveals a cluster whose small, compact appearance suggests it lies much farther away than the pair of clusters Hubble imaged. Moreover, this compact cluster lies along the same line of sight -- though not necessarily at the same distance -- as a quasar that resides 10 billion light-years from Earth.

If the cluster, which seems to show star-lit fragments of infant galaxies, indeed lies 10 billion light-years away, it would represent the most distant cluster ever observed, Dressler says. He adds that while Hubble's impaired optics can't resolve the shapes of galaxies in this grouping, the cluster's very blue appearance hints that it contains many star-forming systems.

Lennox L. Cowie of the University of Hawaii in Honolulu notes that galaxies in dense clusters may evolve more rapidly, and perhaps far differently, than the greater number of galaxies in the universe at large. In fact, he says, some observations hint that elliptical galaxies outside of clusters might become spirals, rather than the other way around. Cowie speculates that over different time scales, today's collection of galaxies in and outside of clusters may ultimately have resulted from the merger of millions of minigalaxies (SN: 7/11/92, p.22).