URI KESHET ET AL.

Scientists discovered the universe's background of gamma rays--the most highly energized form of light known--three decades ago. But the origin of this radiation remains a matter of debate. Some astronomers propose that it emanates from individual quasar-type galaxies, known as blazers, which are powered by supermassive black holes. A second theory posits that galaxy clusters are the source because matter drawn toward them at extremely high speeds can collide with cosmic microwave background light and excite photons to gamma-ray energies. Now new findings, to be published in the Astrophysical Journal, support the contention that galaxy clusters are the bulk source of gamma rays in the universe.

Caleb Scharf of Columbia University and Reshmi Mukherjee of Barnard College analyzed nine years of data collected by NASA's Compton Gamma Ray Observatory. By comparing this information with an inventory of 2,469 galaxy clusters, they determined that the sky surrounding the most massive clusters contains higher quantities of gamma rays than other regions do. "The more massive the cluster, and greater the gravitational potential, the brighter the gamma-ray halo [is]," Mukherjee explains. The brightness they observed, she adds, matches well with predictions from the so-called Loeb-Waxman theory of gamma ray production. Future forays into deep space by observation crafts such as the European Space Agency's INTEGRAL in October and NASA's GLAST in 2006 should help scientists resolve galaxies' gamma-ray haloes in greater detail and shed further light on the origins of these mysterious rays.