Dramatic dust-swaddled stellar nurseries seem to be the main sources of a diffuse background light found in all directions, an Antarctic balloon experiment has revealed. The results could help illuminate how star formation has changed over the history of the universe.
Astronomers have long suspected that individual galaxies are responsible for a diffuse glow of long-wavelength infrared light, called the far infrared background, that was detected by NASA's Cosmic Background Explorer satellite in the 1990s.
But accounting for all that light has been difficult, because astronomers must look for such distant galaxies in submillimetre light, which sits between radio and infrared light in the electromagnetic spectrum. Water vapour in Earth's atmosphere easily absorbs this radiation, making it difficult to detect from the ground.
The Balloon-borne Large Aperture Submillimeter Telescope (BLAST) got around this problem by floating above most of Earth's atmosphere. The 2-metre telescope flew above Antarctica for 12 days and landed in early 2007.
By comparing BLAST's data with that of the Spitzer Space Telescope, a team has identified some 450 individual sources that seem to be responsible for almost all of the far infrared background in the patch of sky BLAST observed.
Intense star formation
"Essentially all the radiation came from individual galaxies," says Mark Devlin of the University of Pennsylvania in Philadelphia.
More than half of the light seems to have been created by distant starbursts – galaxies undergoing intense star formation – at a time when the universe was less than 5 billion years old. Smashups between galaxies triggered the formation of the most extreme bursts, which can radiate 1000 times more light than the Milky Way.
Dust in these distant galaxies blocks visible light from reaching Earth, but the energy of newly forming stars is absorbed and re-radiated in the infrared part of the spectrum.
"It seems finally we have identified where most of the dust in the universe is," says Asantha Cooray, an astrophysicist at the University of California, Irvine.
Since stars rely on dust and gas for fuel, this could be useful for mapping how stars formed over the universe's history. The Milky Way converts the equivalent of just 4 times the mass of the Sun into new stars each year, Devlin says, while that figure is about 1000 for some galaxies observed in this study.
BLAST's detector is a match for one that will launch aboard the European Space Agency's Herschel Space Observatory later this year. Herschel, which will be larger than Hubble, will be able to map dusty galaxies over larger areas of the sky and see galaxies that were beyond BLAST's range.
Journal reference: Nature (vol 458, p 737)
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