Resolving a 30-year-old mystery, astronomers say they have identified the source of the gamma-ray background, the faint high-energy radiation permeating the cosmos. Caleb A. Scharf and Reshmi Mukherjee of Columbia University compared known positions of 2,469 galaxy clusters with data collected for 9 years by NASA’s Compton Gamma Ray Observatory. Gamma rays are concentrated in regions surrounding the most massive clusters, the scientists report in an upcoming Astrophysical Journal.
This statistical result supports a theory proposed recently by Avi Loeb of Harvard University and Eli Waxman of the Weizmann Institute in Rehovot, Israel.
According to their model, when matter began organizing into the filaments, sheets, and clusters of galaxies that fill the cosmos, the gas gravitationally lured into these regions became compressed and heated. The resulting shock waves produced high-speed electrons, which in turn collided with photons from the cosmic microwave background, the low-energy glow left over from the Big Bang. These collisions pumped up some of the microwave photons to gamma-ray energies, producing the gamma-ray background.
“In the past, galaxy clusters were found to emit X rays, but this is the first observational indication that they emit gamma rays,” says Loeb. The finding suggests a new way to detect galaxy clusters, he adds.
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