Dark Matter Found In Galactic 'Ghosts'
(From the March/April 1999 issue of StarDate magazine)
Dark galactic "cannonballs" may greatly outnumber large, bright galaxies like our own Milky Way, says John Kormendy of the University of Hawaii Institute for Astronomy. But these small, dense galaxies are so dark that they are almost impossible to find.
Kormendy and Kenneth Freeman of Mount Stromlo Observatory in Australia analyzed 43 galaxies, from bright giants like the Milky Way to galaxies with a tiny fraction of the mass of the Milky Way. They found that the smallest, faintest galaxies contain the highest percentage of "dark matter" -- material that produces no energy, but that can be identified from its gravitational pull on the matter around it. Dark matter may account for 90 percent of the total mass of the universe.
Dark matter appears to account for about half of the mass of most large galaxies, but up to 99 percent of the mass of the smallest galaxies. Kormendy says it's logical to assume that there are galaxies where dark matter accounts for even more of the total mass. "There might be a population of dark galaxies that contain too few stars to be discovered," he said.
"Are there galaxies that are so faint that we can't see them? I think the answer is probably yes," says Vera Rubin, the astronomer who first proposed the existence of dark matter. "Nature tends to do anything that is not forbidden [by the laws of physics]."
The material in such galaxies is packed together more tightly than in bright galactic giants like the Milky Way. "They're like cannonballs," says Kormendy. They contain only a few million stars, and span a few hundred light-years. By comparison, the Milky Way contains hundreds of billions of stars and its spiral-shaped disk is about 100,000 light-years across.
Dark dwarf galaxies probably formed when the universe was only a few million years old -- less than one percent of its current age. When they were born, they contained very little raw material for the formation of stars. When the first generation of stars in these galaxies exploded, the blasts cleared away the remaining gas, depriving the galaxies of a source for new stars. Only the dark matter remained.
Our own galactic neighborhood, called the Local Group, is dominated by small, dark galaxies. A team of astronomers from the National Optical Astronomy Observatories and Johns Hopkins University announced the discovery of another galactic dwarf in the Local Group, named Andromeda VI (the sixth dwarf known to orbit the giant spiral galaxy in Andromeda). It lies about 900,000 light-years from the Andromeda Galaxy and 2.6 million light-years from Earth.
Scientists have not yet determined the composition of dark matter; it may be a mixture of "normal" objects, like black holes or other dead stars, and previously unknown types of subatomic particles. "It's like we're in a black room doing an all-black puzzle," said Rubin. "When we do find a piece, we don't know exactly where it fits." -- — Damond Benningfield
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