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Mapping the Milky Way’s dark matter
Posted: 06 January 2010

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The dark matter halo that surrounds the Milky Way Galaxy, making up 70 percent of its mass, appears to be shaped like a squashed ball, according to new findings presented today at the meeting of the American Astronomical Society in Washington, DC.

Streams of stars are pulled out of satellite galaxies by the gravity of the Milky Way.

Even though we cannot see it, smell it, touch it or taste it, dark matter does make its presence felt on other objects through its gravity. For the dozens of small, satellite galaxies that buzz around the Milky Way, this massive dark matter halo is a very real presence, its gravity pulling and tugging on the satellites, ripping stars from them to form a stellar stream that rings the galaxy like ribbon.

Astronomers Dr David Law of the University of California, Los Angeles, Dr Steven Majewski of the University of Virginia and Dr Kathryn Johnston of Columbia University, decided to track one of these satellite galaxies – the Sagittarius Dwarf – back along its residual stellar stream to accurately map its orbit around the Milky Way. What they found, however, was that the Sagittarius Stream flowed oddly, following a path that it shouldn’t have if the dark matter halo around the Milky Way was perfectly spherical.

A diagram showing the apparent ‘squashed beachball’ shape of the Galaxy’s dark matter halo.

The only explanation that made sense was if the dark matter halo was shaped like a flattened ball squashed on one side, with differing lengths in all three dimensions (i.e. it is ‘tri-axial’). Rather surprisingly, the fact that it is squashed on its side means that the dark matter halo must be aligned at a right angle to the spiral disc of the Milky Way.

The team plan to backtrack along other stellar streams belonging to various satellite galaxies to see if they also follow this geometry for the dark matter halo. “In the meantime, such a squashed dark matter halo is one of the best explanations for the observed data,” says Law. “We expected some amount of flattening based on predictions of the best dark matter theories, but the extent, and particularly the orientation, of the flattening was quite unexpected. We’re pretty excited about this, because it begs the question of how our Galaxy formed in its present orientation.”

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