Mysterious "Dragons" Make Universe's Gamma Ray Fog

If ancient mariners were mapping the universe, the gamma ray fog that fills the cosmos would now be marked with a warning: Here be dragons.

That's the conclusion of a new study of the fog, which found that the source of the high-energy radiation is even more of a mystery than anticipated.

Previously, scientists had surmised that most if not all of the fog's gamma rays are being created by powerful galaxies with active supermassive black holes at their hearts.

Active black holes spew jets of particles traveling at nearly the speed of light. When these high-speed particles slam into ambient gases, gamma rays are born and go zipping in all directions through interstellar space.

But with the Fermi Gamma-ray Space Telescope, scientists now have a way to measure the contribution of black hole jets directly.

"And it turns out to be only 30 percent at most," astrophysicist Marco Ajello, of the Kavli Institute for Particle Astrophysics and Cosmology in California, said today at a press briefing.

That means no one knows where the rest of the fog's gamma rays are coming from, and for now there's no obvious candidate in sight. Taking a cue from medieval mapmakers, the Fermi team has dubbed the unknown gamma ray sources dragons.

Gamma Ray Mapping

Gamma rays are the most energetic forms of light. In space the rays are produced by violent events, such as supernovae, and by high-energy sources such as neutron stars, pulsars, and active black holes.

A gamma ray burst from a supernova, for example, can unleash more energy in just ten seconds than the sun will over its ten-billion-year lifetime. (Related: "Gamma Ray Burst Caused Mass Extinction?")

In the late 1960s orbiting observatories began revealing a ubiquitous background of gamma rays permeating the universe. Today, scientists looking at the sky with "gamma-ray vision" see a blanket of light bisected by the galactic plane, with brighter spots marking various gamma ray sources.