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.
(Related: "New Probe's First Gamma-Ray Sky Map Unveiled.")
But the idea that no one can find the main source of the gamma ray fog is "incredible," said Martin Weisskopf, a project scientist for the Chandra X-ray Observatory at NASA's Marshall Space Flight Center who was not involved in the study.
"The important thing here is that there is a population of gamma-ray sources out there that one cannot identify, and that's very exciting," Weisskopf said from Hawaii's Big Island, where the study was presented during a meeting of the High Energy Astrophysics Division of the American Astronomical Society.
Finding the Dragons' Lairs
Future Fermi data may allow scientists to pinpoint the dragons' lairs. But for now, astronomers are grappling with several theories to explain the gamma ray fog.
Accelerated particles coming from star-forming galaxies or from large clusters of merging galaxies are two possibilities. (See a picture of a supernova shock wave thought to be acting as a particle accelerator.)
Gamma rays may be "produced by the shocks as the great clusters of galaxies are being formed, [by] dark matter, or by spinning neutron stars," Weisskopf said.
In general, studying gamma rays and their sources can help scientists understand the high-energy events involved in the births and deaths of stars and galaxies, which may ultimately aid studies of the origins of the universe.
So no matter what the dragons turn out to be, Weisskopf said, by unraveling the fog's secrets, "we will certainly learn more about how the universe is put together."