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Bright spot. The most distant known galaxy (arrow) may have lit up the universe's Dark Ages.

Dark Age Galaxy

Astronomers have discovered a galaxy dating from just 780 million years after the big bang, making it the oldest object ever seen. The find suggests that stars and galaxies began lighting up the universe even earlier than astronomers and cosmologists had supposed.

The universe hasn't always twinkled with stars. After the big bang, it took more than 300,000 years for cold gas to begin to amalgamate into the first stars and galaxies. Only after those stars ignited and began cooking away the surrounding gas did the so-called Dark Ages lift and stars and galaxies become fully visible.

Any infant galaxy dating from the end of the Dark Ages is likely to be at an immense distance and therefore very faint. In an online paper to be published in the 1 April issue of Astrophysical Journal Letters, Esther Hu of the University of Hawaii, Manoa, and colleagues describe how they found one such galaxy using a natural image intensifier: a gravitational lens. They trained the 10-meter Keck telescopes atop Mauna Kea, Hawaii, on a cluster of galaxies called Abell 370, about 6 billion light-years away. The gravity of this cluster acted as a lens, bending the light from a more distant galaxy behind it and brightening it.

Hu's team looked for signs of hydrogen, a signature of star formation, suitably stretched by redshift during its passage across the universe. The galaxy Hu and her team found has a redshift of 6.56, putting it about 15.5 billion light-years away, so this light dates back to just 780 million years after the big bang. Because neutral hydrogen gas would absorb such a signal, the observation means that neutral hydrogen must have already been burned away by other galaxies, Hu says. "The end of the Dark Ages lies earlier in time than people had previously thought," says Hu.

"It's an important paper--as long as the results hold," says theoretical astrophysicist Abraham Loeb of Harvard University. Loeb is reluctant to pin too much on a single emission from a single faint source. But Hyron Spinrad, an astronomer at the University of California, Berkeley, is confident that Hu's faint source is a true early galaxy. "I think the result is right," he says.

Related sites
Esther Hu's home page
Institute for Astronomy Web site
Keck Observatory Web site