Amid controversy about its future in orbit (ScienceNOW, 9 March), the Hubble Space Telescope has captured what may become its defining image: the Ultra Deep Field, the most sensitive photograph of the distant universe ever taken. The image, released today, contains about 10,000 objects--many of which existed when the cosmos was less than 1 billion years old.
The original Hubble Deep Field photo, taken in 1995, launched a new era of research into how galaxies evolve. But it also tantalized astronomers with hints of the true building blocks of modern galaxies, just beyond Hubble's grasp at the time. Thanks to the new Advanced Camera for Surveys, installed 2 years ago by astronauts during the space shuttle's last servicing mission to Hubble, the telescope can now detect those ancient objects. The instrument is sensitive to near-infrared light, the wavelengths at which the emissions of extremely distant galaxies--stretched by the expansion of space--shine most brightly.
The telescope focused on a small square of the sky in the Southern Hemisphere for nearly 300 hours to create the photo, which is stunningly sharp even by Hubble's aesthetic standards. Among the galaxies are hundreds of tiny, ill-formed blotches of stars that should help astronomers devise a coherent picture of how galaxies assembled after the big bang, says project leader Steven Beckwith, director of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. "We're clearly seeing back to a time when the universe was chaotic," he says. "Almost all of these are galaxies or things that will become galaxies, but we see a variety of unusual shapes that we can't yet identify."
Independent data from Hubble's camera, using a different observing technique, and from a ground-based telescope in Chile already verify that some of the objects shone when the universe was just 800 million to 1 billion years old. Other images of the Ultra Deep Field with a cryogenically cooled infrared camera and spectrograph on Hubble, called NICMOS, may have exposed even more primitive objects: protogalaxies that coalesced and started forming stars just 500 million years after the big bang.
The Ultra Deep Field will be the best portrait of typical galaxies growing within the young cosmos for at least a decade, says STScI astronomer Massimo Stiavelli. NASA's cancellation of a final planned servicing mission to Hubble means that a newer camera will not examine the same patch of sky more deeply in infrared light, potentially exposing even fainter objects. "It's a big loss," Stiavelli says. Still, he notes, the Ultra Deep Field "will be one of Hubble's legacies for a long time."