The ever-present fog of energetic gamma rays permeating the universe isn't created by what astronomers expected, new observations from NASA's Fermi Gamma-ray Space Telescope reveal, leaving scientists with a new cosmic mystery to solve.
The sky glows in gamma rays even far away from well-known bright sources, such as pulsars and gas clouds within our own Milky Way galaxy or the most luminous active galaxies. Conventionally, astronomers thought that the accumulated glow of active galactic nuclei — black hole-powered jets emanating from active galaxies — accounted for most of this gamma-ray background.
"Thanks to Fermi, we now know for certain that this is not the case," said Marco Ajello, an astrophysicist at Kavli Institute for Particle Astrophysics and Cosmology, jointly located at SLAC National Accelerator Laboratory and Stanford University in California.
For now the scientists are representing the unknown sources with a dragon symbol – something that ancient mapmakers used to represent areas of the world not yet explored.
Ajello and other Fermi scientists used data from the telescope's first year in action and compared the emission from active galaxies that Fermi detected directly against the number needed to produce the observed extragalactic background.
Active galaxies possess central black holes containing millions to billions of times the sun's mass, known as supermassive black holes. 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.
But it turns out that active galactic nuclei are only minor contributors to the background gamma ray glow.
"Active galaxies can explain less than 30 percent of the extragalactic gamma-ray background Fermi sees," Ajello said. "That leaves a lot of room for scientific discovery as we puzzle out what else may be responsible."
Ajello presented his findings Tuesday at a meeting of the American Astronomical Society's High-Energy Astrophysics Division in Waikoloa, Hawaii. A paper on the findings has been submitted to The Astrophysical Journal.
What the mystery majority contributors are isn't yet known, though the team has a few ideas.
Particles accelerated both in star-forming galaxies and galaxy cluster mergers could contribute to the gamma ray background, the scientists said.
And dark matter – the mysterious substance that neither produces nor obscures light but whose gravity corrals normal matter – is always a possibility.
"Dark matter may be a type of as-yet-unknown subatomic particle. If that's true, dark matter particles may interact with each other in a way that produces gamma rays," Ajello added.
Improved analysis and extra sky exposure will enable the Fermi team to address these potential contributions.
But what the unknown source turns out to be isn't the important part of the finding, said Martin Weisskopf, Chandra Project Scientist from NASA's Marshall Space Flight Center who isn't involved with the Fermi team. What matters is that "there is apparently a population of gamma ray sources out there that one cannot identify," he said.
Launched in June 2008, the Fermi space telescope is continuously mapping the sky in gamma ray wavelengths and looking for so-called gamma ray bursts, the brightest explosions in the universe.
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