Rosy Outlook on Oldest Galaxies

Hidden in a corner of the nondescript patch of sky called the Hubble Deep Field, astronomers have found what are almost certainly the farthest and oldest galaxies ever seen. So distant that the expansion of the universe has stretched their light all the way into the infrared region of the spectrum, they may have formed just a few hundred million years after the universe itself. If the universe was born 13 billion years ago, they are probably 12.3 billion years old.

The discovery, announced today at a NASA press conference, is a follow-up to the original Deep Field exposure by the Hubble Space Telescope (HST) in 1996. In an exposure lasting 10 days, the HST soaked up light from that patch of sky, revealing a swarm of blue, silver, and gold galaxies 11.7 billion years old. Those galaxies originally shone brightly in ultraviolet light because of the hot young stars populating them, but the expansion of the universe has "reddened" the ultraviolet into visible wavelengths. Even more distant galaxies, reddened all the way to the infrared, would have eluded the original Deep Field observation.

Last January, Rodger Thompson of Steward Observatory at the University of Arizona, Tucson, and his team went looking for those galaxies by aiming HST's infrared camera, called NICMOS, at one-eighth of the Deep Field for 36 hours. In a corner of the Deep Field that held over 300 galaxies in visible light, NICMOS found 100 more. The light from most of those appeared to have been reddened by dust, not great distance; but the light of 10 of the dimmest ones seemed to have been stretched all the way from ultraviolet to infrared, giving them redshifts of 5.0 to 7.0. That would make them the oldest, farthest objects known. "We must be getting very close to seeing the first galaxies formed," said Alan Dressler, an astronomer at the Observatories of Carnegie Institution in Pasadena, California.

"Next we have to sort them out and find out what they are," said Thompson. For now, these 10 ancient objects are too dim for anyone to see what shape they are, estimate how quickly they're forming stars, pinpoint their distance, or even decide whether they're small galaxies or pieces of galaxies. One of them, Thompson thinks, looks a little like an edge-on spiral galaxy and another like a small elliptical. But their identities won't be certain until they're observed with HST's successor, the Next Generation Space Telescope, to be launched in 2007.

Posted in Space