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Gamma-ray glow
steeped in mystery

Posted: 4 March 2010

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An omnipresent fog of high energy gamma-ray radiation that bathes the entire Universe is being produced mostly by a mysterious, unknown source, revealed scientists this week at a meeting of the American Astronomical Society’s High-Energy Astrophysics Division in sunny Hawaii.

Colourful star-forming regions litter the Large Magellanic Cloud. Could such regions be adding to the extragalactic gamma-ray glow? Image: C Smith/S Points/MCELS Team/NOAO/AURA/NSF.

Gamma-rays are the most energetic type of radiation possible. Prior to the latest findings by NASA’s Fermi Gamma-Ray Space Telescope, it had been assumed that this extragalactic fog was produced almost entirely by the action of fast-moving jets of charged particles emanating from black holes in active galaxies. Not so, reveal Marco Ajello and Markus Ackermann of the Kavli Institute of Particle Astrophysics and Cosmology in California. Using the Large Area Telescope onboard Fermi, they carefully measured the background extragalactic glow of gamma-rays, carefully separating it from foreground sources such as pulsars, supernovae remnant and star-forming regions.

“The extragalactic background is very faint, and it is easily confused with the bright emission from the Milky Way,” says Ackermann. With the background isolated in their studies (these background measurements are published in the 12 March edition of the journal Physical Review Letters), Ajello and Ackermann then began to compare it with observations of relatively nearby active galaxies with black holes (called active galactic nuclei, or AGN). Supermassive black holes at the centres of galaxies, when fed large amounts of matter, can whip up jets of charged particles with the help of powerful magnetic fields, blasting out from the cores of galaxies at close to the speed of light. When particles in the jet strike a photon of visible or infrared light, they cause the photon to jump up in energy and become a gamma ray. Alternatively, should one of the jet particles smash into the nucleus of an atom floating around in a gas cloud, the collision can create a subatomic particle called a pion, which quickly decays into two gamma rays.

A map of the gamma-ray sky, as constructed by FermiÕs Large Area Telescope. The blue regions above and below the Milky Way indicate the extragalactic fog. Image: NASA/DOE/Fermi LAT Collaboration.

The assumption has always been that the gamma-ray background is caused by the accumulation of gamma rays from many active galaxies too distant to be resolved. But by measuring the gamma-ray production in nearby AGN such as within the giant elliptical galaxy M87, they found that the amount of gamma rays they produced was too small; there was no way there could be enough AGN to support the background extragalactic glow at energies between 0.1 and 100 billion electronvolts. In fact, it seems that AGN contribute only 30 percent.

So where is the other 70 percent coming from? “Particle acceleration occurring in normal star-forming galaxies is a strong contender, and so is particle acceleration during the final assembly of the large scale structure we observe today, for example where clusters are merging together,” suggests Ackermann. There is physical evidence to back up the star-forming galaxies theory. In November 2009 it was announced that Fermi had discovered gamma-ray emission from local starburst galaxies including M82, NGC 253 and from the Tarantula Nebula in the Large Magellanic Cloud, the largest star-forming region in our Local Group of galaxies. The emission of the gamma rays seen by Fermi matched the locations of the gas clouds in the Tarantula Nebula seen in visible light (see our related story here )

There is also a wild card theory to consider – if dark matter turns out to be some unknown subatomic particle in the standard model, says Marco Ajello, then collisions between two dark matter particles could result in the release of gamma rays. Until we know more about dark matter, it is not something we can rule out.

The findings will be published in an upcoming issue of The Astrophysical Journal.

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