Astronomers have known for years that a powerful energy source at the core of the Milky Way is sending gamma-rays out through the entire galaxy, but now they're puzzling over an entirely new phenomenon--a halo of gamma radiation that appears to embrace the galaxy's perimeter. The discovery of this halo, thousands of light-years in diameter, was announced today at a meeting of the American Astronomical Society in Estes Park, Colorado. Astrophysicists don't know what the source is yet, but they have no shortage of theories.
Gamma-rays, the most intense form of radiation, are usually produced only by violent astronomical events, such as the explosion of a star. The first gamma-ray map of the sky, generated 2 years ago by a space telescope called the Compton Gamma-Ray Observatory, was dominated by gamma-rays coming from the center of the Milky Way, where intense star formation occurs, and by a few bright sources outside our galaxy.
Dave Dixon, a physicist at the University of California, Riverside, made a remarkable find when he subtracted from this image the core glow of the Milky Way and other known gamma-ray sources. The remaining gamma-rays--about 10% of those observed--emerged from a huge, diffuse cloud surrounding the galaxy, from a direction where no high-energy source is known to exist.
How can gamma-rays be produced in what looks like empty space? "If you were to poll the people at the conference, the starburst theory would be the most popular" explanation, Dixon says. In this theory, rapidly expanding bubbles of cosmic rays, produced by supernova explosions inside the galaxy, "pop" and release energy when they run into low-energy infrared light outside the galaxy. According to another exotic theory, the gamma halo is emitted by "dark matter" around the galaxy--invisible stuff that some cosmologists believe makes up 90% of the universe.
Other theories, no doubt, will be proposed. "We all sat around at dinner last night and were racking our brains on how to explain it," says Neil Gehrels of the Goddard Space Flight Center in Greenbelt, Maryland, the project scientist for the Compton mission. "Once we get away from the galaxy, we don't know any sources" violent enough to produce the halo. He suggests there may be "some kind of galactic accelerator" at work. To find this accelerator in the sky, astronomers may have to wait for the next generation of gamma-ray telescopes, due to be launched a decade from now.