Speedy galactic aluminum vexes astronomers

Astronomers rarely make discoveries that blast a hole in their understanding of the galaxy. Now though, explosive new findings are forcing them to rethink the structure of the Milky Way.

 Using gamma-ray spectrometers mounted on high-flying balloons, some researchers have turned their attention to radioactive aluminum associated with clouds of gas near the center of the galaxy. Astronomers believe that this material was ejected by either a supernova or a giant star in the region.

 Instead of seeing gamma rays with aluminum's characteristic energy, which would have indicated that the metal moves at the same speed as neighboring galactic material, they saw emissions with a broad distribution of energies around the expected value. This smearing indicated that the cloud of aluminum was expanding at 450 kilometers per second, three times faster than anticipated. The astronomers, from NASA's Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland in College Park, report their findings in the Nov. 7 Nature.

"This raises important questions about what our galaxy is made of," said Goddard's Jack Tueller, a study coauthor. Astronomers had thought that material ejected by supernovas or giant stars would come to rest after about 100,000 years, slowed down by shock waves and collisions with other interstellar material. Instead, they calculate from the size of the cloud and its rate of expansion that it has rocketed around for 750,000 years as if little stood in its way. "Don't worry," says Tueller. "It's nowhere near us."

 The researchers guess that the aluminum may be racing through an unusually low density pocket of the interstellar medium. A region of space populated by many supernovas may have been blasted free of other gas by the explosions, leaving the way clear for an aluminum-containing cloud.

 Alternatively, the metal may have precipitated out of its gaseous form. If the aluminum took the form of billions of tiny dust grains, "they would pass through space like little bullets," says Tueller. "Dust would shoot through shock waves set up by the gas around it with no braking." For this to be true, he adds, much more dust than predicted must be produced by decaying stars.

"These are very exciting observations," says astronomer Richard Lingenfelter of the University of California, San Diego, who believes the dust explanation best matches current knowledge. "It's a big candy store from the theoretical viewpoint," he says, pointing to recent studies suggesting that metallic particles accelerate cosmic rays.

 If supernovas produce more dust than expected, according to Tueller, it could mean that the ratio of gas to dust in the galaxy has been miscalculated for decades. Some meteorites bear signs of dust impacts that may confirm this interpretation, he adds. "But if this explanation is right, it means we've underestimated how much of the galaxy is made of dust for a long time."

 The spreading aluminum should interest astronomers for years, says Reuven Ramaty of Goddard. "The nuclear formation of material is central to astronomy -- and to humanity. After all, we're all made of the ashes of stars."