Nearby galaxy sheds light on dark matter.

Astronomers have measured for the first time the movement across the sky of a galaxy other than our own. Using the new data, which characterize the orbit of the Milky Way's nearest galactic neighbor, California researchers have made what they consider the most accurate estimate to date of the amount and shape of dark matter in our galaxy Dark matter

-- hypothetical material thought to make up 90 to 95 percent of the mass in the universe -- doesn't emit light, yet it exerts a gravitational tug.

According to the new calculations, the mass of the Milky Way contains five to 10 times as much dark matter as visible stars. Moreover, the team infers that this unseen material lies in a giant halo at least six times larger than the visible disk of the galaxy. A halo this big would not only dwarf the visible Milky Way, it would extend far beyond the Milky Way's nearest neighbor and satellite galaxy, the Large Magellanic Cloud (LMC).

While astronomers had already derived similar estimates for the size and mass of dark matter in the Milky Way, "this measurement is much more reliable than all previous measurements;' says Douglas N.C. Lin of the University of California, Santa Cruz. He and his Santa Cruz colleagues, Burton E Jones and Arnold R. Klemola, reported the work this week at a meeting of the American Astronomical Society in Berkeley, Calif.

Since dark matter can't be seen, astronomers use its gravitational influence on visible objects as an indirect means of detecting it. To search for dark matter in the Milky Way, therefore, Lin and his coworkers decided to study the motion of the LMC, bound to our galaxy in an orbit that takes it above and below the Milky Way's disk. Many astronomers had previously measured the LMC's radial velocity, or motion along the line of sight to the Milky Way. But Lin's team sought its transverse velocity- its motion across the sky- which no one had ever measured for any galaxy but our own.

Such motion is extremely difficult to detect because at a great distance from Earth, even a speedy galaxy appears to cross the sky at a snail's pace. In their study, the astronomers compared photographs, taken 15 years apart at the Cerro Tololo Inter-American Observatory in La Serena, Chile, of a star-packed region at the LMC's northeastern edge.

Using an electronic scanner, they could discern the slight shifts in position, relative to a fixed background of distant galaxies, of 250 LMC stars during those 15 years. Based on this analysis, the researchers report that the satellite galaxy moves across the sky at a brisk 220 kilometers per second. To keep the LMC gravitationally bound to the Milky Way despite this transverse velocity, the team calculates, our galaxy must contain at least as much mass as 600 billion suns about five times as much mass as the visible galaxy,

Deriving the shape of the proposed halo of dark matter surrounding the Milky Way requires further knowledge of the LMC's orbit. Lin's team reconstructed the galaxy's likely path by examining a huge comet-like stream of hydrogen gas that astronomers believe was torn from the LMC by the Milky Way's gravity Lin suggests that the stream trails the LMC and marks the galaxy's path at the time of its previous revolution around the Milky Way, about 2.5 billion years ago. "The stream provides a tracer, just as a let [contrail] shows where a jet used to be;' he notes.

Lin notes that segments of the hydrogen stream have a large radial velocity Since this velocity had to have originated from the stream's parent galaxy, the LMC, this indicates that the galaxy takes a highly elliptical, rather than circular, path around the Milky Way

A distribution of dark matter that extends far beyond the LMC best explains the galaxy's elliptical path. The LMC lies about 170,000 light-years from the center of the Milky Way In comparison, the Milky Way's halo of dark matter may have a minimum radius of 300,000 light-years, Lin says, and could extend as far as 800,000 light-years.

Scott D. Tremaine of the University o! Toronto says the study is important but that its implications for measuring dark matter are far from clear-cut. "Most people will probably be a little cautious in applying this; it's easy to underestimate [errors] in determining dark matter. It's not like [measuring] the orbit of a planet, which we can study as it goes around many times. Here you have just one snapshot."

Lin says the study indicates that the LMC and perhaps other satellite galaxies of the Milky Way are about 1 to 2 billion years younger than the oldest stars in our galaxy. Bit by bit, the Milky Way's gravity is tearing apart the LMC and will gobble it up entirely in about 20 billion years, he adds.