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Gamma ray "mystery" not related to dark matter
Posted: JULY 10, 2009

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The distribution of certain gamma rays in the Milky Way, speculated by some to be evidence of dark matter, can instead be explained by the way antimatter positrons move through the Galaxy.

It is well known that our Galaxy is filled with tiny subatomic particles known as positrons, the antimatter counterpart of typical, everyday electrons. When an electron and positron encounter each other in space the two particles annihilate, that is they disappear, and their energy is released as gamma rays.

"These positrons are born at nearly the speed of light, and travel thousands of light years before they slow down enough in dense clouds of gas to have a chance of joining with an electron to annihilate in a dance of death," explains James Higdon, a physics professor at the Claremont Colleges.

Image: X-ray (NASA/CXC/CfA/E.O'Sullivan); Optical (Canada-France-Hawaii-Telescope/Coelum)

In the last five years, gamma ray measurements collected by ESA's space satellite INTEGRAL have perplexed astronomers because the distribution of these gamma rays across different parts of the Milky Way galaxy was not as expected: about 50 percent larger in the inner bulge of the Milky Way than the outer disc. This lead some scientists to believe that dark matter was the culprit. While undetectable, dark matter is thought to make up a large proportion of the mass of the Universe, and its presence is inferred from gravitational effects on visible matter such as stars and galaxies.

New interpretation of the gamma ray data suggests that their distribution can instead be explained by the way antimatter positrons from the radioactive decay of elements, created by massive stellar explosions, propagate through the galaxy.

"The observed distribution of gamma rays is consistent with the standard picture where the source of positrons is the radioactive decay of isotopes of nickel, titanium and aluminum produced in supernova explosions of stars more massive than the Sun," says Richard Rothschild, a research scientist at UC San Diego.

"There is no great mystery," adds Richard Lingenfelter, also of UCSD. "The observed distribution of gamma rays is in fact quite consistent with the standard picture."

Lingenfelter explains why the bulge-to-disc annihilation ratio is about twice as large as that expected for positron production from the decay of radioactive nuclei ejected from supernova explosions. "About half of the radioactive decay positrons born in the disc escape into the halo and annihilate there rather than in the disc, while most of those born in the bulge don't escape and die there," he tells Astronomy Now. "This explains the higher observed bulge-to-disc ratio of positron annihilation compared to positron production."

Furthermore, the the basic assumption of dark matter decays is flawed because it assumes that the positrons annihilate very close to the exploding stars from which they originated. "We clearly demonstrated this was not the case, and that the distribution of the gamma rays observed by the gamma ray satellite was not a detection or indication of a 'dark matter signal'," says Lingenfelter.

The new results are published in this week's issue of Physical Review Letters and in last month's Astrophysical Journal.