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New stellar streams discovered in Andromeda
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Gamma ray "mystery" not related to dark matter
DR EMILY BALDWIN
ASTRONOMY NOW
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.

2010 Yearbook
Our latest 132-page Astronomy Now special edition is an extravaganza of astronomy for the year ahead, with a complete 30-page guide to observing the planets, moon, meteor showers, two solar eclipses, and the deep sky in 2010.
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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
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3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
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Infinity Rising
This special publication features the photography of British astro-imager Nik Szymanek and covers a range of photographic methods from basic to advanced. Beautiful pictures of the night sky can be obtained with a simple camera and tripod before tackling more difficult projects, such as guided astrophotography through the telescope and CCD imaging.
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Starry Night
Explore the Universe with these new versions of the award-winning Starry Night Software. Available now from the Astronomy Now Store.
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Exploring Mars
Astronomy Now is pleased to announce the publication of Exploring Mars. The very best images of Mars taken by orbiting spacecraft and NASA's Spirit and Opportunity rovers fill up the 98 glossy pages of this special edition!
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Mars rover poster
This new poster features some of the best pictures from NASA's amazing Mars Exploration Rovers Spirit and Opportunity.
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