Gamma-ray glow bathes Milky Way.

A mysterious halo of gamma rays not associated with any known celestial objects extends thousands of light-years from the core of the Milky Way and may surround the entire galaxy, astronomers report.

"These gamma rays are providing the first evidence that some sort of high-energy process is occurring at large distances from the galactic core," says physicist David D. Dixon of the University of California, Riverside.

Analyzing data from the Earth-orbiting Compton Gamma Ray Observatory, Dixon and his collaborators have mapped the strength and extent of the gamma-ray halo. He presented the findings this week at a meeting of the High-Energy Astrophysics Division of the American Astronomical Society in Estes Park, Colo.

Another assessment of satellite data, slated for publication in the Astrophysical Journal early next year, also reveals the presence of an extended halo, says David L. Bertsch of NASA's Goddard Space Flight Center in Greenbelt, Md. Bertsch is an author of the forthcoming study.

The gamma rays forming the halo have energies up to a billion times that of visible light. One possible explanation of the gamma rays' origin, Dixon says, is that they are created when electrons traveling near the speed of light collide with lower-energy, infrared photons. Cosmic rays are a source of such electrons, and an aura of dim infrared photons around the Milky Way like those found recently around similar spiral galaxies could provide "seed photons."

The gamma-ray distribution may also provide indirect evidence of dark matter--the universe's missing mass, whose existence scientists have inferred but not yet demonstrated-Dixon says. According to one theory, some massive, dark-matter particles occasionally collide with each other and either generate gamma rays or produce particles that decay into gamma rays.

"The most likely explanation [for the extended halo] is that cosmic rays are boosting lower-energy photons to gamma-ray strength," asserts Bertsch.