Quasar light points to younger galaxies.

Quasar light points to younger galaxies

Astronomers have usually assumed that most galaxies formed early in the history of the universe, perhaps as much as 10 billion years ago. Two researchers now say they have strong evidence that gas clouds were still coalescing into galaxies as recently as just a few billion years ago. In fact, their observations of the effect of intervening gas clouds on the passage of light from distant quasars to Earth suggest that star formation within nascent galaxies may have reached a peak about 5 billion years ago rather than at an

earlier time.

"It may be that there never really was an epoch of galaxy formation," says Donald G. York of the University of Chicago, who presented the new evidence at last week's American Astronomical Society meeting an Albuquerque, N.M. "It looks as if galaxies are created at different times in different places."

To detect pockets of early star formation associated with the collapse of vast gas clouds, York and Brian Yanny first examined the spectra of distant quasars, looking for the characteristic imprint left when light passes through a large cloud of molecular hydrogen. Such clouds make their presence felt by absorbing certain well-defined wavelengths of light.

York and Yanny found such clouds, but direct observation of a forming galaxy is tricky. Normally, the faint light emitted by clouds just starting to collapse into galaxies would be lost in the background light of the night sky. To overcome that problem, the Chicago team developed a technique for filtering out all but a narrow band of wavelengths of light typically emitted by hot, bright, young stars. By applying this technique to the clouds they identified by quasar light, they could map each cloud's star-formation regions.

"No one had really thought to apply this trick," York says. "But if you block out most of the skylight, suddenly all these galaxies come popping out at you."

The resulting images show pockets of star formation, looking like bright sparks scattered against a dark background. "We may be seeing the first bursts of light from the extended gas clouds that eventuany create fully developed galaxies," York says. Each patch of light apparently represents a small, collapsed piece of cloud that had reached a density large enough to initiate star formation.

By measuring how long it takes light from these collapsing clouds to reach Earth, astronomers can figure out what galaxies looked like billions of years ago and deduce when galaxies started to form.