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Distant Starbursts Light Up the Sky--We Just Can't See Them

A survey from on high confirms the far-flung source of a celestial infrared background
BLAST, FIRB, balloon astronomy, telesccope, infrared background



Mark Halpern

The first results from a balloon-hoisted telescope that floated more than 20 miles (30 kilometers) above Antarctica have shed light on a major source of the celestial background, the electromagnetic radiation—both visible and invisible—that pervades the universe.

The far-infrared background (FIRB), first detected in the mid-1990s by the now-defunct COBE (Cosmic Background Explorer) satellite, is voluminous if not luminous—its many photons have wavelengths much too long to be visible to the human eye. (Infrared radiation is longer in wavelength than visible light, and far-infrared radiation is the long-wavelength portion of that spectrum.) On top of that, Earth's atmosphere is opaque to much of the FIRB spectrum, making a conclusive identification of its source by ground-based telescopes difficult.

By probing the infrared and the longer-wavelength submillimeter regimes from a vantage point high in the stratosphere in 2006, the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) was able to resolve this bountiful background and confirm its source as vigorous star-producing galaxies, most of which are many billions of light-years away, that are obscured from view by enveloping shrouds of dust. The light from these energetic galaxies is absorbed by the dust, which heats up and reradiates it in the infrared.

The FIRB is "a huge player in the energetics of the universe," says cosmologist Mark Devlin of the University of Pennsylvania, BLAST's principal investigator and lead author of the study published last week in Nature. The energy of the far-infrared background is roughly equivalent to the entire optical background—all that we can see in the background sky in visible wavelengths, according to Devlin. "The difference is that in the optical you can go look and see where it's coming from—there's a star, there's a galaxy—so you can associate the background with individual objects," he says.

The source of the infrared background was "not a huge mystery," Devlin is careful to note, but it could not be broken down without a dedicated search such as that performed with BLAST. The telescope sampled photons at far-infrared and submillimeter wavelengths of 250, 350 and 500 microns, just above the FIRB's intensity peak of about 200 microns, and NASA's Spitzer Space Telescope provided additional space-based observations at shorter infrared wavelengths. (A micron is a millionth of a meter, or around 40 millionths of an inch.) The emissions from these so-called starburst galaxies peak at about 100 microns at the source, but the wavelengths are stretched, or redshifted, to far-infrared and submillimeter lengths as the photons make their way across an expanding universe over billions of years.

With the combination of surveys, Devlin and his colleagues were able to resolve hundreds of individual sources and conclude that young, energetic galaxies in the early universe provide the bulk of the FIRB. "Now we know the details of the story," he says.

That story helps illuminate basic models of how stars and galaxies form. "The FIRB is intimately linked to star formation, and therefore to galaxy formation," says study co-author Enzo Pascale, a cosmologist at Cardiff University in Wales. Such structure formation, Pascale says, is one of the least defined aspects of the universe's development. "Understanding how galaxies have formed in our universe," he says, "requires studying the light they emitted while forming, which is the FIRB."

Ian Smail, an astrophysicist at Durham University in England who wrote an accompanying commentary in Nature, agrees, noting that one of the basic parameters in theoretical models of galaxy formation is "how much light has been emitted from all the stars which have ever been formed." To find that tally, Smail adds, "you need to account for the half [of the light] that is missing from the optical and near-infrared because it was absorbed by dust."

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