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Searching for evidence of ringing galaxies.

Strike a bell, and it rings. Perturb a galaxy, and it, too, should oscillate. Computer models in which hundreds of thousands of particles attract each other according to the law of gravity indicate that a small disturbance can make these particles move in a coordinated fashion somewhat like the motion of a vibrating bell.

This finding suggests that galaxies, which typically contain billions of stars and large amounts of interstellar gas and dust, may also vibrate in characteristic ways. Such galactic oscillations, if they exist, would probably have originated in the violent processes that led to galaxy formation.

Now, researchers are developing a catalog of effects that astronomers can look for when seeking to determine whether galaxies actually oscillate. Richard H. Miller of the University of Chicago, working with Bruce F. Smith of NASA's Ames Research Center in Mountain View, Calif., and his coworkers, described some of these potentially observable effects at a meeting of the American Astronomical Society's Division on Dynamical Astronomy held last week in Washington, D.C.

The oscillations represent an important, fundamental dynamical process that previously had not been suspected in galaxies, much less taken into account, Miller contends. However, "there's no smoking gun yet to prove they exist." Miller and his colleagues created their simulated galaxies using powerful computers at the Numerical Aerodynamic Simulation Facility at Ames. Their numerical experiments typically involved tracking the behavior of about 400,000 particles distributed in a sphere.

The researchers found two characteristic oscillations, or fundamental modes, that can be large enough to produce observable effects in galaxies. In both cases, simulations indicate that a single galactic oscillation would take from tens of millions to hundreds of millions of years and that the oscillations could go on undiminished for billions of years.

In the first fundamental mode, the entire galaxy expands and contracts, with material farther from the center moving faster than material closer to the center. In a rotating spiral galaxy, such a radial motion would alter the velocities of hydrogen atoms distributed in vast sheets moving with the galaxy.

Obtaining detailed radio measurements of these velocities and interpreting the numbers is no simple matter, however. In checking six candidate galaxies, Gregory R. Roelofs, now at Philips Research in Palo Alto, Calif., found it difficult to differentiate between effects due to oscillations and those caused by a tilted or slightly twisted flat disk.

In the second fundamental mode, particles closer to the center move in the opposite direction of those farther away, creating an intermediate region where no particles move. This type of motion either piles up material at a galaxy's center to produce a bulge or takes it away to create a hole. The fact that bulges and holes in different galaxies sometimes have comparable dimensions supports this scenario, Miller says. The researchers are now studying how galactic disks respond to oscillations in a surrounding halo of dark matter.
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Title Annotation:observable effects of galaxy oscillations described
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Apr 27, 1996
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