Mapping the universe.

Thanks to the keen eye of the Hubble Space Telescope, astronomers may have accounted for virtually all of the sources of visible light in the universe. That's what Michael S. Vogeley of Princeton University concludes after analyzing variations in the faint background glow between galaxies in the Hubble Deep Field, the tiny patch of sky that the telescope stared at for 2 weeks in late 1995. These Hubble observations are the most detailed survey of galaxies ever taken.

The smoothness of the background--varying by as little as 0.1 percent throughout the Hubble Deep Field--suggests that there are relatively few galaxies fainter than the ones Hubble detected, says Vogeley. Had there been many galaxies too faint for the telescope to detect as individual objects, they would have shown up as large fluctuations in the brightness of seemingly blank regions of the sky.

"We appear close to completing a [visible-light] census of the universe," Vogeley says. The new results suggest that "most of the visible light in the universe hails from galaxies that Hubble can detect."

Vogeley's analysis also indicates that the substantial infrared background revealed in new sky maps (SN: 1/10/98, p. 20) can only be explained by an entirely new population of galaxies, one that can't be seen in visible light. If so, several infrared telescopes now being developed should provide a new window on the universe, he notes.

The Infrared Space Observatory (ISO), launched 2 years ago, is already opening that window. By staring into the far reaches of space through a small, relatively dustfree region of our galaxy, ISO has found 30 far-infrared point sources. David L. Clements of the Institut d'Astrophysique Spatiale in Orsay, France, and his colleagues say that number is 10 times higher than can be accounted for by galaxy counts recorded by an earlier satellite at shorter infrared wavelengths.

The team says the sources probably represent a new population of distant, dusty galaxies that are churning out stars at an enormous rate. Their starlight is presumably absorbed by the surrounding dust, which reemits (D the radiation in the mid infrared. The expansion of the universe then shifts that light to the far infrared, where ISO detects it.

Clements and his team haven't obtained spectra of these point sources yet, so the distance to these putative galaxies is unknown. They are planning further observations with ISO in other parts of the sky to determine whether the galaxies are "just the tip of the iceberg," he says. If infrared instruments larger than ISO find many more such populations, "it would mean that an awful lot of the action in the universe has been missed," Clements says.