14. Invisible Galaxies Found
When an astronomer discovers a new comet, it's named after him, so perhaps we should name 90 percent of the universe in honor of Neal Dalal of the University of California at San Diego and Christopher Kochanek of the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. The two have reported the most direct evidence yet for the invisible mass that has gone missing from cosmological surveys.
Scientists have concluded that the vast bulk of the universe consists of exotic dark matter, a substance that emits no light. Dark matter cannot be observed, obviously, but it should generate a gravitational tug that shapes the behavior of collections of galaxies. Under its influence, moderate-size galaxies such as our own should be surrounded by swarms of smaller galaxies. "The region around the Milky Way should look like the Coma cluster," Kochanek says, referring to a famous, dense grouping of galaxies. Instead, the Milky Way has just two prominent satellites.
Dalal and Kochanek hypothesized that the satellite galaxies might be there but might be dark. To find out, they studied distant galaxies whose light has been distorted by the gravitational pull of other, nearer galaxies. Such distortions often create multiple images of the more remote object; the brightness of each image depends on the distribution of mass around the intervening galaxy. The researchers found that some duplicate images are brighter than others. Those variations denote a lumpy gravitational field, which indicates the intervening galaxies are surrounded by small, unseen companion galaxies. The companions probably consist almost entirely of dark matter.
Kochanek says the Milky Way, too, may be surrounded by small galaxies that lack the ordinary matter needed to make stars. "If you throw out all the gas and stars, you've only lost 10 percent of the mass," he says. "From the point of view of their gravitational effects, it doesn't matter if you get rid of the normal matter." — Jeffrey Winters
