Two of NASA's Great Observatories, the Spitzer and Hubble SpaceTelescopes, have teamed up to "weigh" the stars in several distantgalaxies. One of these galaxies, among the most distant ever seen,appears to be unusually massive and mature for its place in the younguniverse.
This came as a surprise to astronomers. The earliest galaxies in theuniverse are commonly thought to have been much smaller associations ofstars that gradually merged to build large galaxies like our Milky Way.
"This galaxy, named HUDF-JD2, appears to have 'bulked up' amazinglyquickly, within the first few hundred million years after the big bang.It made about eight times more mass in stars than are found in our ownMilky Way today, and then, just as suddenly, it stopped forming newstars," said Dr. Bahram Mobasher of the Space Telescope ScienceInstitute, Baltimore, and the European Space Agency, Paris.
The galaxy was pinpointed among approximately 10,000 others in asmall patch of sky called the Hubble Ultra Deep Field. The galaxy isbelieved to be about as far away as the most distant known galaxies. Itrepresents an era when the universe was only 800 million years old.That is about five percent of the universe's age of 14 billion years.
Scientists studying the Ultra Deep Field found this galaxy inHubble's infrared images. They expected it to be young and small, likeother known galaxies at similar distances. Instead, they found evidencethe galaxy is remarkably mature and much more massive. Its stars appearto have been in place for a long time.
Hubble's optical-light Ultra Deep Field image is the deepest imageever taken, yet this galaxy was not evident. This indicates much of thegalaxy's optical light has been absorbed by traveling billions oflight-years through intervening hydrogen gas. The galaxy was detectedusing Hubble's near-infrared camera and multi-object spectrometer. Itwas also detected by an infrared camera on the Very Large Telescope atthe European Southern Observatory. At those longer infraredwavelengths, it is very faint and red.
The big surprise is how much brighter the galaxy is inlonger-wavelength infrared images from the Spitzer Space Telescope.Spitzer is sensitive to the light from older, redder stars, whichshould make up most of the mass in a galaxy. The infrared brightness ofthe galaxy suggests it is massive. "This would be quite a big galaxyeven today," said Dr. Mark Dickinson of the National Optical AstronomyObservatory, Tucson, Ariz. "At a time when the universe was only 800million years old, it's positively gigantic."
Spitzer observations were also independently reported by Dr.Laurence Eyles from the University of Exeter in the United Kingdom andDr. Haojing Yan of the Spitzer Science Center, Pasadena, Calif. Theyalso revealed evidence for mature stars in more ordinary, less massivegalaxies at similar distances, when the universe was less than onebillion years old.
The new observations reported by Mobasher extend this notion ofsurprisingly mature "baby galaxies" to an object which is perhaps 10times more massive, and which seemed to form its stars even earlier inthe history of the universe.
Mobasher's team estimated the distance to this galaxy by combininginformation provided by the Hubble, Spitzer, and Very Large Telescopeobservations. The relative brightness of the galaxy at differentwavelengths is influenced by the expanding universe and allowsastronomers to estimate its distance. They can also get an idea of themake-up of the galaxy in terms of the mass and age of its stars.
While astronomers generally believe most galaxies were builtpiecewise by mergers of smaller galaxies, the discovery of this objectsuggests at least a few galaxies formed quickly long ago. For such alarge galaxy, this would have been a tremendously explosive event ofstar birth.
JPL manages the Spitzer Space Telescope mission for NASA. Scienceoperations are conducted at the Spitzer Science Center at theCalifornia Institute of Technology in Pasadena.The Hubble SpaceTelescope is a project of international cooperation between NASA andthe European Space Agency. The Very Large Telescope is a project of theEuropean Southern Observatory at the Paranal Observatory in Atacama,Chile.
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