Astronomers have discovered a strange dark mass far away in the universe: what appears to be a huge cluster of galaxies that's almost completely undetectable, except for the x-rays it's shedding. The finding, described in tomorrow's issue of Nature, may constitute a new kind of cosmic object.
Makoto Hattori of Tohoku University in Sendai, Japan, and his colleagues pointed Japan's ASCA satellite in the direction of a quasar whose light is bent by the gravity of an immense concentration of mass--presumably a cluster of galaxies--lying between the quasar and Earth. Previous studies of this "gravitational lens" using optical telescopes had turned up only a single visible galaxy. But ASCA's x-ray detector found vast amounts of hot, x-ray-emitting gas, much like the gas that envelops ordinary clusters.
What's more, the spectrum of the x-rays showed that this gas cloud, some 10 billion light-years away, is rich in iron. That finding makes the absence of visible galaxies in the cluster even more puzzling, because iron and other heavy elements are produced in stars, which usually congregate in galaxies--and the stars are nowhere to be found. The researchers conclude in their report that they have witnessed a "new type of astronomical object."
The discovery "proves once again that bright galaxies are not reliable signposts of where matter in the universe is distributed," says Richard Mushotzky, an x-ray astronomer at NASA's Goddard Space Flight Center in Greenbelt, Maryland. He points out, however, that the cluster may contain many faint galaxies not picked up by optical telescopes. "A deep image from the Hubble Space Telescope could help look for those," he says.