Hubble lakes ultralong look at the cosmos.

Astronomers this week unveiled the deepest visible-light portrait of the universe ever made. Compiled by the Hubble Space Telescope as it stared into a narrow corridor of space more than 13 billon light-years long, the mosaic of images also includes infrared pictures of what appear to be the most distant objects detected so far.

Dubbed the Hubble Ultra Deep Field (UDF), the images feature a panoply of galaxy shapes, from the classic spirals and ellipticals common in the cosmos today to a zoo of misshapen oddballs that may be among the first galaxies to have coalesced. Follow-up studies to measure just how remote these galaxies are may require a new generation of telescopes, but some of these bodies could hail from a time when the 13.7-billion-year-old universe was only about 300 million years old, the UDF astronomers say.

Located in a region of the Fornax cluster examined by a slew of other telescopes, UDF "takes us to within a stone's throw of the Big

Bang itself," says Massimo Stiavelli of the Space Telescope Science Institute in Baltimore. His team released the pictures to eager astronomers worldwide on March 9. "This is like the great land rush, where the gun is fired, and everyone goes off[at once]," says institute director Steven Beckwith.

The images were released as astronomers and members of Congress objected to NASA plans to cancel a shuttle mission to repair and upgrade the telescope (see page 170).

With 11.3 days of observing time spread over 4 months, UDF can depict objects as faint as the glow of a firefly on the moon. It reveals galaxies only one-fourth as bright as the faintest recorded by Hubble's previous deep-field studies (SN: 11/28/98, p. 343).

The telescope's Advanced Camera for Surveys contributed the visible-light portion of UDF, which spans a sky region equivalent to one-hundredth the apparent area of the full moon. Hubble's recently revived Near Infrared Camera and Multi-Object Spectrograph covered about half that area for UDF. That camera detects the most-remote galaxies because cosmic expansion shifts the light emitted by distant galaxies into the near-infrared part of the spectrum.

Some of the galaxies that UDF detected hark back to the end of the cosmic Dark Ages, the era when the first stars reheated and reionized the universe, which had been cooling down since the Big Bang. "The great sensitivity of the UDF allows us to find and study typical objects [from that era] rather than the most extreme, brightest ones," notes Stiavelli.

Distant galaxies recorded in previous surveys appear to be small, but astronomers have argued that these bodies may merely represent the central, brightest parts of much bigger galaxies (SN: 5/27/00, p. 348). By detecting fainter emissions, UDF may provide an important test of this hypothesis.

Christopher Conselice of the California Institute of Technology in Pasadena says he plans to use UDF to chronicle how most galaxies formed 6 billion to 11.5 billion years after the Big Bang. The Hubble images will offer new clues about how the modern cosmos came to be, he says.--R. COWEN