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Hubble gets multiple views of distant galaxy.

When hunting the most distant galaxies, astronomers often have to content themselves with but a single image. Now, scientists have obtained a trio of images of one of these extraordinarily faint, starlit bodies.

Taken with NASA's Hubble Space Telescope, the portraits range in wavelength from visible light to the near-infrared and provide an unprecedented look at the shape and rate of star formation of one of the most distant objects known in the universe. "It is fair to say this is the best-characterized very distant galaxy so far," says Daniel Stern of the University of California, Berkeley.

Ray J. Weymann and Lisa J. Storrie-Lombardi of the Carnegie Institution of Washington in Pasadena, Calif., and their colleagues, including Stern and Hyron Spinrad of Berkeley, describe their work in an article posted on the Internet (http://xxx.lanl.gov/abs/astro-ph/ 9807208).

The galaxy lies in an extensively studied patch of sky known as the Hubble Deep Field (HDF). The Hubble Telescope observed the region in visible light in late 1995 and surveyed about one-third of it in the near-infrared this January.

Analyzing images of the galaxy, dubbed HDF 4-473.0, Weymann and his colleagues strongly suspected that it is distant. The galaxy did not show up in a picture taken in yellow light, appeared only as a dim blob in an image at slightly longer wavelength, and was considerably brighter viewed in near-infrared light. That's indicative of a galaxy whose short-wavelength emissions can't be seen because they are absorbed by hydrogen gas, which is plentiful between Earth and distant galaxies.

The astronomers then used the Keck I Telescope atop Hawaii's Mauna Kea to obtain spectra of the galaxy, which revealed that it is more than 12 billion light-years from Earth. This makes HDF 4473.0 the second most distant galaxy known (SN: 5/2/98, p. 280). Because the light left the galaxy so long ago, the images provide a snapshot of the object when the cosmos was less than 1 billion years old.

Spinrad notes that several other galaxies glimpsed at that early epoch appear to be composed of two or more parts. In contrast, says Weymann, HDF 4473.0 "seems not only quite compact but also quite regular, as if it had not just been assembled but had been around for a while." It would be puzzling if so mature a galaxy was present when the cosmos was still in its infancy.

Alternatively, the galaxy may have irregular features that are too faint for Hubble to have discerned, notes Mark Dickinson of Johns Hopkins University and the Space Telescope Science Institute, both in Baltimore.

The HDF 4-473.0 galaxy is about half the size of ordinary, nearby galaxies. It appears to be making the equivalent of 13 suns per year, a star-formation rate typical of young, distant galaxies, Stern says.

Unlike other extremely distant galaxies, this one was found by a systematic analysis of images, he adds. With this method, "we can now start estimating the star formation rate ... in the early universe and compare it to the values [today]."

If astronomers hope to use Hubble's near-infrared camera to do so, they'll have to hurry. The camera is expected to run out of coolant and cease operation in December.
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Author:Cowen, Ron
Publication:Science News
Article Type:Brief Article
Date:Aug 8, 1998
Words:540
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