Like every self-respecting actor or pop singer, our galaxy has an entourage. Astronomers have identified at least eight pint-sized galaxies that trail the Milky Way and consist almost entirely of dark matter. The finding helps resolve a curious shortage of such galaxies.
Current theory predicts that large galaxies such as the Milky Way should be surrounded by hundreds of much smaller satellite galaxies. These should have formed when hydrogen gas accumulated around small clumps of dark matter and then condensed to make stars. But astronomers had been unable to find more than a handful of these small bodies. That "missing satellite" problem has led some researchers to question the theories that predict the abundance of tiny galaxies. Others say the little devils are out there but are hard to see because they are so faint--and perhaps even starless.
The new result suggests that the satellite galaxies are out there after all. Marla Geha of the Herzberg Institute of Astrophysics in Victoria, British Columbia, and Joshua Simon of the California Institute of Technology in Pasadena focused on eight candidate objects that had already been located by the Sloan Digital Sky Survey, which uses a 2.5-meter telescope at Apache Point, New Mexico, to survey a quarter of the sky. They studied the light from the objects using a spectrometer on the 10-meter Keck II telescope on Mauna Kea, Hawaii. When they calculated the motions of the bodies' stars, they confirmed that the eight not only were dwarf galaxies--the smallest ever seen--but also were as much as 99%% dark matter. Geha and Simon report in the 10 November issue of The Astrophysical Journal that the dwarf galaxies' stars orbit at speeds of only a few kilometers per second, compared to the sun's 200 kilometers per second or so. That's slow but fast enough to reveal the gravitational strength of dark matter.
"Our observations produced the first mass measurements for most of these galaxies," Simon says, demonstrating they are "in fact composed almost entirely of dark matter." He notes that even large galaxies such as the Milky Way are up to 90%% dark matter, but the higher percentage in the dwarfs could be related to their size--on average, only about one ten-thousandth the mass of the Milky Way. Perhaps, Simon says, when stars explode as supernovae inside a dwarf galaxy, the force might "blow much of the gas that is present completely out of the galaxy, reducing the amount of normal matter it contains while leaving the dark matter in place."
It's a "very important" study, says astronomer Taft Armandroff, director of the W. M. Keck Observatory. He notes that the finding "dramatically increases the number of dwarf galaxies surrounding our Milky Way galaxy" and "firmly establishes the dominance of dark matter over luminous matter in these very small galaxies," as predicted by theory. Further proof involving other galaxies will have to wait, however, Armandroff says. Although similar surveys have been attempted with the Andromeda galaxy, our galaxy's nearest neighbor, "we cannot yet study its dwarf galaxy companions to the same level of detail," he says.
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