Image: Hubble Ultra Deep Field
The Hubble Ultra Deep Field (HUDF) is a million-second-long exposure that reveals the first galaxies to have emerged from the so-called "dark ages" of the universe.
updated 3/9/2004 2:16:02 PM ET 2004-03-09T19:16:02

Astronomers using the Hubble Space Telescope (HST) unveiled the deepest look into the universe yet, a portrait of what could be the most distant galaxies ever seen.

The new image, called the Hubble Ultra Deep Field (HUDF), includes objects that until now have been too faint to be seen and includes ancient galaxies that emerged just 700 million years after the Big Bang from what astronomers call the "dark ages" of the universe.

"This image is the deepest view in the visible that we've ever taken, where an object about as bright as a firefly on the Moon would be visible," said Massimo Stiavelli, of the Space Telescope Science Institute (STScI) in Baltimore and the UHDF project leader.

Stiavelli said the new image is six times more sensitive than previous deep sky surveys and four times better than even Hubble's last faraway looks, the Hubble Deep Fields (HDFs), taken in 1995 and 1998.

"It has these extra colors with extra red shifts, which leads you to the end of the dark ages, something you couldn't do with the HDF," he added.

Behind the Hubble Ultra Deep Field
The HUDF field contains an estimated 10,000 galaxies in a patch of sky in the constellation Fornax, a region just below the constellation Orion, that appears in an area of the sky that appears largely empty if observed by ground-based instruments. The image is about one-tenth the diameter of the full moon and took Hubble one million seconds to take. To cover the entire sky with such detail would take the HST one million years, astronomers said.

The HUDF is the result of two separate exposures, one taken by Hubble's Advanced Camera for Surveys (ACS) and the other by the Near Infrared Camera and Multi-object Spectrometer (NICMOS). By combining the two, astronomers can search for galaxies that existed between 800 and 400 million years after the Big Bang. But it's the NICMOS instrument that will reveal the farthest galaxies ever seen, because only it can detect light stretched past the visible, which is ACS territory, far into the near-infrared spectrum.

Astronomers can tell how old a galaxy is by measuring the light it emits, specifically the amount of light that has been shifted toward the red end of the spectrum. The higher red shift a galaxy has, the more distant it is and the earlier it existed in the universe.

Hubble researchers are confident their new image contains galaxies whose light has been stretched to a red shift of 6 or more. There's even a good case that it contains ancient galaxies of red shift 12, which would place them about 300 million years after the Big Bang.

Mario Livio, head of the Institute Science Division at STScI, says that if red shift 12 galaxies are indeed in the image, they will be found soon. "It could happen this afternoon," Livio told in an interview prior to the Hubble announcement. "That might be stretching it a bit, but it will be easy."

Stiavelli, head of ultra deep field observations, said that finding a red shift 12 galaxy will be important because it will be done not with a gravitational lens, but "by brute force." Gravitational lenses, which use the gravitational fields of nearby objects to magnify those in the background, have helped ground observers detect galaxies dating back to just 500 million years post-Big Bang. But the natural zoom view is rare.

STScI astronomers said the ACS image contained a wide range of regular galaxies, including spirals, ellipticals, as well as a number of oddball galaxies, some of which appear to be interacting with one another. Some of the galaxies look like toothpicks, while others are arranged like links on a bracelet.

"We can now see tidal tails, wisps of gas, that cover large distances," Stiavelli said, adding that there are some interaction effects that defy identification, include some very red, very strange objects. "We don't know exactly what this is."

Hubble's Hits"This will have a lot of impact on the James Webb Space Telescope (JWST), since it's tasked with studying this period at even higher sensitivity," said Rodger Thompson of the University of Arizona and the NICMOS principal investigator. Scheduled for launch in 2011, the JWST is to be Hubble's successor and pick up where the HST left off.

Hubble's ACS picture required a series of exposures taken over the course of 400 HST orbits around Earth from Sept. 24, 2003, to Jan. 16, 2004. About the size of a phone booth, ACS captured ancient photons of light that began traversing the universe even before Earth existed. Photons of light from the very faintest objects arrived at a trickle of one photon per minute, as opposed to millions of photons per minute from nearer galaxies.

The entire HUDF was observed with the advanced camera's "grism" spectrograph, an instrument used to measure distances to these distant objects.

Using what time is left
"We hope, in the next few months, to not get much sleep and analyze this data to see what's happening in the universe," said Sangeeta Malhotra of the STScI and principal investigator for the Ultra Deep Field's ACS grism follow-up study. Already, the grism spectra has yielded the identification of about 1,000 objects, including some prime candidates for distant galaxies, which appear as intensely faint, red points of light.

Based on those identifications, some of these objects are among the farthest and youngest galaxies ever seen, said Malhotra, adding that the grism spectra also distinguish among other types of very red objects, such as old and dusty red galaxies, quasars and cool dwarf stars.

Astronomers are eager to see the Hubble receive a stay of execution in the form of future servicing missions by NASA's space shuttles to extend the telescope's lifetime. Adam Riess, a supernova researcher for STScI, said an extension could help astronomers find supernova early in the universe's lifetime.

"There are no supernovae in this deep field, but the results show that supernova in the early universe could be found if Hubble could be extended," Riess said. "Those could provide valuable insight into dark energy and fate of the universe."

The STScI is operated by the Association of Universities for Research in Astronomy, Inc. under contract with NASA's Goddard Space Flight Center, Greenbelt, Md. The HST is a project of international cooperation between NASA and the European Space Agency.

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