Space and Earth Sciences News

Image 1: Fermi's LAT detects gamma-rays by tracking the electrons and positrons they produce after striking layers of tungsten. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab Image 2: Fermi data invalidates a once-popular explanation for the extragalactic gamma-ray background. Jets from active galaxies play only a minor role in producing the emission. Credit: NASA/DOE/Fermi LAT Collaboration [ Download PDF Version ]Image 3: This view of the gamma-ray sky is constructed from one year of Fermi Large Area Telescope (LAT) observations. The blue color includes the extragalactic gamma-ray background. The map shows the rate at which the LAT detects gamma rays with energies above 300 million electron volts -- about 120 million times the energy of visible light -- from different sky directions. Brighter colors represent higher rates. Credit: NASA/DOE/Fermi LAT Collaboration

"Active galaxies can explain less than 30 percent of the extragalactic gamma-ray background Fermi sees," said Marco Ajello, an astrophysicist at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), jointly located at SLAC National Accelerator Laboratory and Stanford University, Calif. "That leaves a lot of room for scientific discovery as we puzzle out what else may be responsible."
"Thanks to Fermi, we now know for certain that this is not the case," Ajello said.
"The extragalactic background is very faint, and it's easily confused with the bright emission from the Milky Way," said Markus Ackermann, another member of the Fermi LAT team at KIPAC who led the measurement study. "We have done a very careful job in separating the two components to determine the background's absolute level."
"Particle acceleration occurring in normal star-forming galaxies is a strong contender," Ackermann explained. "So is particle acceleration during the final assembly of the large-scale structure we observe today, for example, where clusters of galaxies are merging together."