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Tuned in to new noise from the cosmos

Radio static could be from earliest stars, black holes

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11:41am, January 8, 2009
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LONG BEACH, Calif. — When astronomers launched a balloon-borne experiment from Palestine, Texas three summers ago, they expected to find a faint radio signal from the slight warming of interstellar space by an early generation of stars. Instead, Al Kogut of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleagues discovered a booming, uniformly distributed radio noise six times louder than anyone had predicted.

The team described the mysterious and pervasive radio static January 7 at the winter meeting of the American Astronomical Society. They also posted four reports online detailing their analyses and interpretation of the data at http://arxiv.org/abs/0901.0562, http://arxiv.org/abs/0901.0559, http://arxiv.org/abs/0901.0555 and http://arxiv.org/abs/0901.0546.

The researchers calculate that the radio noise is much too large to be accounted for by the combined emissions of all the galaxies in the universe that emit radio waves. They also suggest that the static could be signals generated by the first supermassive black holes. Cosmologist David Spergel of Princeton University, not a member of the discovery team, says the static could also be from the first generation of stars. “And those are the most conservative explanations,” he adds.

Kogut and his colleagues base their findings on 2.5 hours of data gathered during a flight of seven radio receivers called ARCADE (Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission).

ARCADE’s radio receivers, which were cooled to a temperature just 2.7 degrees above absolute zero for the balloon flight on July 22, 2006, are the first detectors capable of definitively identifying the strange radio signals, Kogut says.

Kogut adds that a retrospective analysis of several other low-frequency radio-wave studies in the 1980s and 1990s hints at the unexpected static.

Because ARCADE operates at the same low temperature as the cosmic microwave background — the whisper of radiation left over from the Big Bang that itself was accidentally discovered as radio noise — heat from the instrument can’t be confused with the radio signals it detects. Emissions from the sky are also compared to an onboard radio-emitting source.

Data from the 36-kilometer-altitude flight, in which ARCADE examined about 7 percent of the sky centered over eastern Texas, reveals a pattern of radio signals that strongly resembles synchrotron emission.

Such emission is generated by electrons accelerated to high speeds by strong magnetic fields. Electrons energized by the maelstrom of activity, including intense magnetic fields, associated with an active supermassive black hole could produce this radiation, notes Spergel. So could star-forming regions, in which massive, short-lived stars explode as supernovas, accelerating charged particles to high speeds, he adds.

Kogut and his collaborators, who include Michael Seiffert of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., don’t know the distance from which the radio signals originate. But the radio static does not match any known pattern from sources in the Milky Way. Nor can it be accounted for by nearby supermassive black holes or other radio sources in nearby galaxies, which are well studied, Kogut says. And a new population of radio-emitting galaxies, too faint to be observed directly, would have to vastly outnumber all the known galaxies in the universe in order to produce such a strong radio signal.

By process of elimination, that leaves some unknown source — possibly the first generation of supermassive black holes or the first stars — from the early universe. The radio spectrum seen by ARCADE “is telling us that we’re actually seeing a signature from a period of time that we know very little about and are very interested in,” says Spergel. A more exotic, less likely possibility, he adds, is radio emission from some new type of elementary particle.

Observations with the proposed Square Kilometer Array, a network of low-frequency radio telescopes astronomers are hoping to build, could more sensitively probe the radio static and possibly resolve the mystery background into individual sources, Spergel says.

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