New twist in the old search for dark matter.

For two decades, astronomers have inferred the presence of galactic dark matter by measuring the velocities of gas and stars orbiting near the visible edge of galaxies. The farther any orbiting material lies outside a known concentration of mass, the slower its velocity. Yet researchers have repeatedly found that the rotational velocity of material at the visible outskirts of galaxies doesn't slow down. Instead, it levels off, indicating that a halo of unseen matter - perhaps black holes or some other type of exotic, invisible mass - extends beyond the visible edge of galaxies, providing the extra gravitational tug needed to keep material orbiting at a constant, high speed.

But such evidence of dark matter says little about its shape or about the overall distribution of mass in galaxies. Now, a group of astronomers reports that the presence of a twisted disk of material, sticking out of the plane of a galaxy, has revealed the shape of dark matter in that galaxy.

Thomas Y. Steiman-Cameron of NASA's Ames Research Center in Mountain View, Calif., and Richard H. Durisen of Indiana University in Bloomington began their study about 10 years ago, using a computer to simulate the evolution of a titled disk of matter, formed when a galaxy's gravity grabs a nearby blob of material. Over time, the inclined disk would become twisted or wraped, depending on the overall distribution of mass in the galaxy - both visible and dark matter. The flatter, or less spherical, the galaxy, the more rapid the twisting.

Steiman-Cameron adds that a disk lying flat in the plane of a visible galaxy can't become twisted and thus can't help define the full three-dimensional structure of the galaxy in which it lies.

While such a model suggested that a study of galaxies with twisted, inclined disks could reveal the shape of dark matter, the researchers hadn't applied the results of their study to any particular galaxy. That state of affairs changed dramatically when a colleague showed them a picture of the galaxy NGC 4753. The unusual pattern of dust lanes in the galaxy, imaged by John Kormendy of the University of Hawaii in Honolulu, matched the dust pattern the researchers had predicted would be produced by their model of a twisted disk, inclined about 15 degrees toward the plane of the visible galaxy.

Though NGC 4753 - twisted disk and all - appears nearly as flat as a slightly bulging pancake, the galaxy's true shape is very different, asserts Steiman-Cameron. Estimating the disk's age and using Kormendy's image to analyze the amount of twisting, he and his research team conclude that NGC 4753 has a nearly spherical shape.

The dark, or unseen, matter in the galaxy must take the shape of a slightly flattened sphere and accounts for the vast majority of the total mass in NGC 4753, report Steiman-Cameron, Durisen, and Kormendy in the October ASTRONOMICAL JOURNAL.

Theorist Scott D. Tremaine of the University of Toronto says he agrees that twisted disks can reveal the geometry of dark matter in a galaxy. But he adds that the details of the team's calculations need to be repeated using other galaxies. "If they had 12 galaxies like this, everyone would sit up and take notice," Tremaine says. "But with just one case, you have to be cautious."

He adds that the near-spherical shape of dark matter inferred by the researchers may challenge theorists, who have postulated that galactic dark matter has a far flatter shape. But he notes that the finding generally agrees with dark matter estimates inferred from observations of polar-ring galaxies -- bodies in which a ring of gas and dust orbits at nearly right angles to the plane of the galaxy.