Spitzer and Hubble Find 'Big Baby' Galaxies in Newborn Universe

Newswise — Two of NASA's Great Observatories, the Spitzer and Hubble Space Telescopes, have teamed up to "weigh" the stars in several very distant galaxies. One of these galaxies, among the most distant ever seen, appears to be unusually massive and mature for its place in the younguniverse. This comes as a surprise to astronomers because the earliestgalaxies in the universe are commonly thought to have been much smaller agglomerations of stars that gradually merged together to build large majestic galaxies like our Milky Way.

"This galaxy appears to have 'bulked up' amazingly quickly, within thefirst few hundred million years after the Big Bang," says BahramMobasher of the Space Telescope Science Institute and the European SpaceAgency, a member of the team which discovered the galaxy. "It made abouteight times more mass in stars than are found in our own Milky Way today,and then, just as suddenly, it stopped forming new stars. It appears tohave grown old prematurely."

The galaxy was pinpointed among approximately 10,000 others in a smallpatch of sky called the Hubble Ultra Deep Field. Thanks to the HubbleSpace Telescope, this area is captured in the deepest images of theuniverse ever made by humankind at optical and near-infrared wavelengths.It is also within the deepest survey from the Spitzer Space Telescope,the Great Observatories Origins Deep Survey (or GOODS). The galaxy isbelieved to be about as far away as the most distant galaxies andquasars now known. The light reaching us today began its journey whenthe universe was only about 800 million years old.

Scientists studying the Ultra Deep Field found this galaxy in Hubble'sinfrared images and expected it to be a very young "baby" galaxy, likeothers known at similar distances. Instead, they found a "teenager," muchbigger than other galaxies known from this young cosmic era, and alreadyquite mature."

Hubble's Advanced Camera for Surveys (ACS) does not see the galaxy at all,despite the fact that the Ultra Deep Field is the deepest image ever takenin optical light. This indicates that the galaxy's blue light has beenabsorbed by traveling billions of light-years through intervening hydrogengas (imagine trying to see the bottom of a silt-laden pond). The galaxy wasdetected using Hubble's infrared camera, the Near Infrared Camera andMulti-Object Spectrometer (NICMOS), and also with an infrared camera on theVery Large Telescope (VLT) at the European Southern Observatory, but atthose wavelengths it is very faint and red.

However, the big surprise was how much brighter the galaxy is in imagesfrom Spitzer's Infrared Array Camera (IRAC), which easily detects itat wavelengths as much as five times longer than those seen by the Hubble.Spitzer's IRAC is sensitive to the light from older, redder stars whichshould make up most of the mass in a galaxy, and the brightness of thegalaxy suggests that it is quite massive indeed. "This would be quite abig galaxy even today," says Mark Dickinson of the National OpticalAstronomy Observatory (NOAO). "At a time when the universe was only 800million years old, it's positively gigantic."

The object is also well detected with Spitzer's Multiband ImagingPhotometer (MIPS) which covers wavelengths fifteeen times longer than thoseof the Hubble, making it sensitive to energetic processes in galaxies.This observation is consistent with the object hosting a supermassiveblack hole at its center, if indeed it is this massive and was formed atthis early stage in the history of the universe.

The GOODS Spitzer observations have previously revealed evidence formature stars in more ordinary, less massive galaxies at similardistances. Lawrence Eyles from the University of Exeter andcollaborators, and Haojing Yan of the Spitzer Science Center,working with other members of the GOODS team, have published jointSpitzer and Hubble analyses that identify other galaxies nearly as massiveas the Milky Way, seen when the universe was less than one billionyears old. The new observations by Mobasher and his colleaguesdramatically extend this notion of surprisingly mature "baby galaxies"to an object which is perhaps ten times more massive, and which seemedto form its stars even earlier in the history of the universe.

Mobasher and his collaborators estimated the distance to this galaxyby combining the information provided by the Hubble, Spitzer, and VLTobservations. Together, these observatories cover a wide swath of theelectromagnetic spectrum, from visible to mid-infrared wavelengths(0.4 to 24 microns). The relative brightness of the galaxy at differentwavelengths is influenced by the expanding universe, and allowsastronomers to estimate its distance. At the same time, they can alsoget an idea of the make-up of the galaxy in terms of the mass and age ofits stars. The team has tried to confirm the distance estimate withspectroscopic measurements from the largest ground-based telescopes,the VLT, Keck, and Gemini observatories, but the object has provento be too faint for such observations. However, thanks to the manywavelengths at which the galaxy has been observed, the color signatureappears to be unique, and the estimates of the distance and mass seemrobust. "While we cannot completely discard other scenarios, this appearsto be the most plausible interpretation, given the available data," saysHenry C. Ferguson, a member of the team.

Astronomers generally believe most galaxies were built up piecewise bymergers of smaller galaxies. However, the discovery of this object suggeststhat at least a few galaxies formed quickly and in their entirety, long ago,as some older theories of "monolithic" galaxy formation have suggested. Forsuch a large galaxy, this would have been a tremendously explosive event, andthe energy from the quick emergence of those stars would have helped reheatthe universe very shortly after it cooled following the Big Bang. This earlyepoch (the first 5 percent of the universe's age) is fertile ground awaitingthe James Webb Space Telescope (JWST), which will have the infrared sensitivityto possibly look all the way back to the very first stars that ignited afterthe Big Bang.

Planned for launch in 2013, the JWST will have the light collectingpower not only to see more distant objects, but to measure theirspectral fingerprints as well, yielding even more reliable distances andchemical composition information.

The Mobasher findings will be published in the December 20, 2005 issue of theAstrophysical Journal. Additional findings with Spitzer by Yan will be publishedin the November 2005 issue of the journal.

Electronic images and additional information are available athttp://hubblesite.org/news/2005/28http://www.spitzer.caltech.edu/Media/releases/ssc2005-19/http://www.spacetelescope.org/news/html/heic0513.htmlhttp://www.astro.caltech.edu/pubout/http://www.noao.edu

The Space Telescope Science Institute (STScI) is operated by theAssociation of Universities for Research in Astronomy, Inc. (AURA),for NASA, under contract with the Goddard Space Flight Center,Greenbelt, MD. The Hubble Space Telescope is a project of internationalcooperation between NASA and the European Space Agency (ESA). JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology (Caltech) in Pasadena. JPL is a division of Caltech.