Could Dwarf Galaxies Yield Clues to Dark Matter?

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"These dwarf galaxies are not much to look at, but they may really alter our fundamental views on the nature of dark matter and, perhaps, even gravity."

Mario Mateo, University of Michigan

The Chandra image of NGC 1569 above, a dwarf galaxy 7 million light years from Earth, shows large hot bubbles, or lobes extending above and below a disk of gas along the equator of the galaxy. Observation allowed scientists to measure for the first time the concentration of oxygen, neon, magnesium, and silicon in the bubbles and the disk. They found that bubbles contain oxygen equal to the oxygen contained in 3 million suns.


For the last 10 million to 20 million years NGC 1569 has been undergoing a burst of star formation and supernova explosions, perhaps triggered by a collision with a massive gas cloud. The supernovas eject oxygen and other heavy elements at high velocity into the gas in the galaxy, heating it to millions of degrees. Hot gas boils off the gaseous disk of the galaxy to form the bubbles, which expand out of the galaxy at speeds of hundreds of thousands of miles per hour.

Dwarf galaxies are much smaller than ordinary galaxies like our Milky Way. Because of their size, they have relatively low gravity and matter can escape from them more easily. This property, combined with the fact that dwarf galaxies are the most common type of galaxy in the universe, makes them very important in understanding how the universe was seeded with various elements billions of years ago, when galaxies were forming.

Could dark matter take the form of dwarf stars, elementary particles including neutrinos, or hypothetical and as yet undetected particles that don't interact with visible light or other parts of the electromagnetic spectrum? 

The answer is "yes." Stars in dwarf spheroidal galaxies behave in a way that suggests the galaxies are utterly dominated by dark matter, University of Michigan astronomers have found.

"These galaxies show a problem right from the center," Mateo said. "The velocity doesn't get smaller. It just stays the same, which is eerie."

"We have more than doubled the amount of data having to do with these galaxies, and that allows us to study them in an unprecedented manner. Our research shows that dwarf galaxies are utterly dominated by dark matter, so long as Newtonian gravity adequately describes these systems," said Matthew Walker, currently at the Harvard Center for Astrophysics.

Because the gravitational force of the visible matter is not considered strong enough to prevent stars from escaping, dark matter is believed to hold them together. Other theories exist to explain these discrepancies, though. For example, modified Newtonian dynamics, Mateo said, proposes that gravitational forces become stronger when accelerations are very weak. While their results align with current dark matter models, Mateo  says they also bolster this less-popular explanation.

"Dwarf galaxies are not much to look at," Mateo continued, "but they may really alter our fundamental views on the nature of dark matter and, perhaps, even gravity."

Stars in dwarf spheroidal galaxies behave in a way that suggests the galaxies are utterly dominated by dark matter. Mateo and post-doctoral researcher Matthew Walker measured the velocity of 6,804 stars in seven of the 14 dwarf satellite galaxies orbiting the Milky Way: Carina, Draco, Fornax, Leo I, Leo II, Sculptor and Sextans. They found that, contrary to what Newton's law of gravity predicts, stars in these galaxies do not move slower the farther they are from their galaxy's core.

Posted by Casey Kazan http://www.ns.umich.edu/htdocs/releases/story.php?id=6125 and http://chandra.harvard.edu

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