A Most Precocious Galaxy

Astronomers have taken a picture of a galaxy at a record-breaking distance. Discovered at the world's largest telescope, the 10-meter Keck on Mauna Kea, the galaxy lies so far out in the expanding universe that the wavelengths of its light have been stretched more than sixfold. In cosmologists' units, the galaxy, called RD1, has a redshift of 5.34, compared to 4.92 for the old record-holder. The light that astronomers are now seeing left the galaxy when the universe was less than 6% of its current age, so it may offer a faint, smudgy portrait of a galaxy in its infancy.

Johns Hopkins University astronomer Arjun Dey and colleagues from Hopkins, the University of California at Berkeley, and the Keck telescope found the new record-holder last December following a systematic search for distant galaxies. While observing one galaxy at a redshift of 4, they noticed the spectral line of hydrogen from a nearby object at a much higher redshift. After 10 hours of exposure, they had collected enough light to confirm the redshift of the galaxy, which is more than a hundred million times fainter than the faintest star visible to the naked eye.

RD1 is too faint for the astronomers to collect a full spectrum, which would give them clues about the nature of its stars and gas. But the galaxy is bright in the ultraviolet wavelengths that are the signature of newly formed stars. Other galaxies that have been found by the hundreds at redshifts between 2 and 4--two billion years or so after the birth of the universe--were also rapidly forming stars. But the faint image of RD1 hints at a difference: While later galaxies are compact, this one appears spread out. "Maybe it's multiple clumps, or maybe it's very diffuse and forming stars over a large volume," Dey says. "It's possible that it's just forming." Otherwise, he says, "it's a very average sort of galaxy."

Theorists trying to explain how galaxies took shape from primordial gas say they don't expect average galaxies to be common in the universe's first billion years. But, says theorist Joseph Silk of Berkeley, "the current theory of galaxy formation is full of holes, so sure, current theory can certainly cope with the odd galaxy at redshift 5." But already, there are signs of even more precocious galaxies. At the Keck telescope, a group from the University of Hawaii and the Institute of Astronomy in Cambridge, England, is now reporting that they have picked up a spectral line--but so far, no image--of a galaxy at a redshift of 5.64.