Black-hole-powered jets create fog of gamma rays

Washington, March 3 : Astronomers, using NASA's Fermi Gamma-ray Space Telescope, have found that black-hole-powered jets from active galaxies are responsible for almost a third of the ever-present fog of gamma rays outside our galaxy.

"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, California.

"That leaves a lot of room for scientific discovery as we puzzle out what else may be responsible," he added.

The sky glows in gamma rays even far away from bright sources, such as pulsars and gas clouds within our own Milky Way galaxy or the most luminous active galaxies.

According to the conventional explanation, this background glow represents the accumulated emission of a vast number of active galaxies that are simply too faint and too distant to be resolved as discrete gamma-ray sources.

"Thanks to Fermi, we now know for certain that this is not the case," Ajello said.

Active galaxies possess central black holes containing millions to billions of times the sun's mass.

As matter falls toward the black hole, some of it becomes redirected into jets of particles traveling near the speed of light.

These particles can produce gamma rays in two different ways.

When one strikes a photon of visible or infrared light, the photon can gain energy and become a gamma ray.

If one of the jet's particles strikes the nucleus of a gas atom, the collision can briefly create a particle called a pion, which then rapidly decays into a pair of gamma rays.

The team analyzed data acquired by Fermi''s Large Area Telescope (LAT) during the observatory''s initial year in space.

Ajello and his colleagues compared emissions from active galaxies that Fermi detected directly against the number needed to produce the observed extragalactic background.

Between energies of 0.1 and 100 billion electron volts (GeV) - or from about 100 million to 30 billion times the energy of visible light - active galaxies turn out to be only minor players.

"Particle acceleration occurring in normal star-forming galaxies is a strong contender," explained Markus Ackermann, another member of the Fermi LAT team at KIPAC who led the measurement study.

"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," he said.