ROSAT data hint at a closed universe.

Using an X-ray satellite to detect that a vast cloud of hot gas envelops a small group of galaxies, astronomers have cal culated that the group must contain a extraordinarily high proportion of dark matter - material that doesn't emit light yet exerts a gravitational force.

If the distribution of dark matter within this group is typical of that in the myriad other small groups of galaxies elsewhere in the cosmos, the universe might h enough mass to halt its expansion an eventually collapse in on itself, says Rich ard F. Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Md.

He and his colleagues, David S. Davis of Goddard, John S. Mulchaey of the Space Telescope Science Institute in Baltimore and David Burstein of Arizona State University in Tempe, reported their findings this week at an American Astronomical' Society meeting in Phoenix.

When Mushotzky and his team asked that the X-ray observatory ROSAT cast its eye on a trio of galaxies known as NGC 2300, the astronomers merely wanted find out why one of the galaxies appeared distorted, as if it had slammed into massive object. Their observation had such low priority on ROSAT's work list that they weren't certain the satellite had ever carried it out until a data tape arrived at Goddard last June.

As they expected, ROSAT detected a hot gas cloud that emits X-rays but not visible light. A collision with hot gas could account for the distorted shape of the spiral galaxy in NGC 2300. But surprisingly the researchers found that the gas cloud was both huge and hot, with a diameter of 1.3 million light-years and an average temperature of 10 million kelvins. To keep such a hot, energetic cloud from flying off into space, the team calculated, NGC 2300 must contain 10 to 30 times as much mass as the total visible mass of its three galaxies.

That range ranks among the highest proportions of dark matter yet inferred for any galaxy system, Mushotzky notes. Until recently, he adds, most researchers searching for dark matter have used visible-light studies, focusing on individual galaxies or clusters of 100 or so galaxies. These researchers have calculated that dark matter exceeds visible matter by a factor of three exceeds visible

But roughly half of all galaxies, including our Milky Way, are members of small groups. So if NGC 2300, which lies 150 million light-years from Earth, isn't some oddball group - and that's a big if, Mushotzky admits-then the dark-matter estimate could mean that the expanding universe has enough mass to eventually collapse back on itself. The single ROSAT measurement, Mushotzky adds, makes it impossible to predict whether the universe is in fact absolutely closed or is poised between expanding forever and undergoing collapse.

Calling the new study "exciting and important," Douglas O. Richstone of the University of Michigan in Ann Arbor cites other studies that support it. Earlier ROSAT findings that many galaxies have collided or merged relatively recently suggest that the universe has a high density Richstone argues. Only a high-density universe would have enough mass to produce large mergers and collisions billions of years after the universe began expanding, he notes.

Nonetheless, cautions Burstein, "One massive group does not a closed universe make." He and his colleagues hope to refine their temperature measurement of the gas cloud using an X-ray detector aboard Astro-D, a Japanese satellite scheduled for launch next month. The team also plans to observe other small galaxy groupings with ROSAT.