The galaxy's large mass and maturity come as a surprise, because experts previously thought that early galaxies in the young universe should be less prominent agglomerations of stars, rather than giant collections of hundreds of billions of stars as populous or more so than the Milky Way.

The researchers are particularly intrigued by the fact that star formation in the galaxy seems to have already been completed. This implies that the bulk of the activity that built up the galaxy had occurred even earlier.

"This is truly a significant object," says Richard Ellis, who is the Steele Family Professor of Astronomy at the California Institute of Technology and a member of the discovery team. "Although we are looking back to when the universe was only 6 percent of its present age, this galaxy has already built up a mass in stars eight times that of the Milky Way.

"If the distance measurement to this object holds up to further scrutiny, the fact such a galaxy has already completed its star formation implies a yet earlier period of intense activity," Ellis adds.

"It's like crossing the ocean and meeting a lone seagull, a forerunner of land ahead. There is now every reason to search beyond this object for the cosmic dawn when the first such systems switched on!"

The galaxy was pinpointed among approximately 10,000 others in a small patch of sky called the Hubble Ultra Deep Field (UDF). It is believed to be about as far away as the most distant galaxies known.

Bahram Mobasher of the Space Telescope Science Institute, leader of the science team, explains, "We found this galaxy in Hubble's infrared images of the UDF and expected it to be young and small, like other known galaxies at similar distances. Instead, we found evidence that it is remarkably mature and much more massive. This is the surprising discovery."

The galaxy's great distance was deduced from the fact that Hubble does not see the galaxy in visible light (despite the fact that the UDF is the deepest image ever taken in optical light).

This indicates that the galaxy's blue light has been absorbed by traveling billions of light-years through intervening hydrogen gas. The galaxy was detected using Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS), and with an infrared camera on the Very Large Telescope (VLT) at the European Southern Observatory. At those near-infrared wavelengths it is very faint and red.

The big surprise is how much brighter the galaxy is in images at slightly longer infrared wavelengths from the Spitzer Space Telescope. Spitzer is sensitive to the light from older, redder stars, which should make up most of the mass in a galaxy. The infrared brightness of the galaxy suggests that it is very massive.

Two other Spitzer observations, one reported earlier by Ellis and his colleagues at the University of Exeter, UK, and the other by Haojing Yan of the Spitzer Science Center, had already revealed evidence for mature stars in more ordinary, less massive galaxies at similar distances, when the universe was less than one billion years old.

However, the new observation extends this notion of surprisingly mature galaxies to an object which is perhaps ten times more massive, and which seemed to form its stars even earlier in the history of the universe.

The team estimated the distance to this galaxy by combining the information provided by the Hubble, Spitzer, and VLT observations. The relative brightness of the galaxy at different wavelengths is influenced by the expanding universe, and allows astronomers to estimate its distance. At the same time, they can also get an idea of the make-up of the galaxy in terms of the mass and age of its stars.

Efforts by Dan Stark, a graduate student at Caltech, using both the giant 10 m Keck and 8 m Gemini telescopes failed to pinpoint the galaxy's distance via spectroscopic methods-the astronomers' conventional tool for estimating cosmic distances.

"We have to admit," says Stark, "that we have now reached the point where we are studying sources which lie beyond the spectroscopic capabilities of our current ground-based facilities. It may take the next generation of telescopes, such as the James Webb Space Telescope and Caltech's proposed Thirty Meter Telescope, to confirm the galaxy's distance."

While astronomers generally believe most galaxies were built up piecewise by mergers of smaller galaxies, the discovery of this object suggests that at least a few galaxies formed quickly and in their entirety long ago. For such a large galaxy, this would have been a tremendously explosive event of star birth.

The findings will be published in the December 20, 2005, issue of the Astrophysical Journal.

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