Astronomers said on Tuesday they have found an unusually massive and mature galaxy in the young universe using NASA's Spitzer and Hubble telescopes.

This galaxy, HUDF-JD2, was pinpointed among approximately 10,000 others in a small patch of sky called the Hubble Ultra Deep Field. It is believed to be as far away as the most distant known galaxies.

The galaxy represents an era when the universe was only 800 million years old, about 5 percent of the universe's age of about 14 billion years, the astronomers said.

This came as a surprise. The earliest galaxies in the universe are commonly thought to have been much smaller associations of stars that gradually merged to build large galaxies like our Milky Way.

But HUDF-JD2 appears to have 'bulked up' amazingly quickly, within the first few hundred million years after the big bang, and then it stopped forming new stars just as suddenly, according to Bahram Mobasher, lead astronomer from the Space Telescope Science Institute.

It made about eight times more mass in stars than are found in our Milky Way today, Mobasher said.

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

Scientists studying the Ultra Deep Field found this galaxy in Hubble's infrared images. They expected it to be young and small, like other known galaxies at similar distances.

Instead, they found evidence the galaxy is remarkably mature and much more massive. Its stars appear to have been in place for a long time.

Hubble's optical-light Ultra Deep Field image is the deepest image ever taken, yet this galaxy was not evident. This indicates much of the galaxy's optical 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. It was also detected by an infrared camera on the Very Large Telescope at the European Southern Observatory. At those longer infrared wavelengths, it is very faint and red.

The big surprise is how much brighter the galaxy is in longer-wavelength infrared images 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 new galaxy suggests it is massive, astronomers said. Spitzer observations also revealed evidence for mature stars in more ordinary, less massive galaxies at similar distances, when the universe was less than one billion years old.

The new observations extend the notion of surprisingly mature "baby galaxies" to an object which is perhaps 10 times more massive, and which seemed to form its stars even earlier in the history of the universe.

Mobasher's team estimated the distance to this galaxy by combining information provided by the Hubble, Spitzer, and Very Large Telescope observations. The relative brightness of the galaxy at different wavelengths is influenced by the expanding universe and allows astronomers to estimate its distance.

Source: Xinhua