FOR RELEASE ON JANUARY 9, 1998

WASHINGTON, Jan. 9 --- It has been 35 years since astronomers discovered clouds of atomic hydrogen moving at peculiar velocities of hundreds of kilometers per second through our Galaxy, and they still don't agree on an explanation for these ``high-velocity clouds.'' The primary reason why high-velocity clouds are so poorly understood is that it is very difficult to determine the distances to the clouds, so astronomers don't know their masses, linear diameters, or densities, and have therefore been unable to figure out their nature and origin. Some astronomers have suggested that the clouds are relatively nearby, perhaps a few hundred light years away, while others think that they are at distances of millions of light years from the Galaxy. However, new observations of high-velocity clouds in other galaxies suggest that they are at an intermediate distance, according to a team of researchers presenting a paper today to the American Astronomical Society meeting in Washington, DC. The researchers have also found a possible connection between the rate of star formation and the amount of high-velocity material in these other galaxies; this will help astronomers to understand the processes that have produced high-velocity clouds in the Milky Way and other galaxies. The team is led by Dr. Eric Schulman, an astronomer at the National Radio Astronomy Observatory in Charlottesville, Virginia, and includes Frances Ockels, an undergraduate at the University of Arkansas at Little Rock who worked with Dr. Schulman this past summer, and Dr. Patricia Knezek, an astronomer at The Johns Hopkins University in Baltimore, Maryland.

Dr. Schulman and his team avoided the problem of uncertain distances by studying high-velocity clouds in other galaxies, where the high-velocity clouds and the bulk of the atomic hydrogen in the galaxies are at the approximately the same distance from us. They used the Arecibo 305-meter (1000-foot) radio telescope in Puerto Rico and the Very Large Array of twenty-seven 25-meter (82-foot) radio telescopes in New Mexico to search for high-velocity atomic hydrogen in fifteen spiral galaxies. The researchers also used data from the Infrared Astronomy Satellite (IRAS) to measure the far-infrared radiation from star-forming regions in the galaxies, and they observed optical radiation from stars and regions of star formation in the galaxies using the Michigan-Dartmouth-M.I.T. 1.3-meter (50-inch) telescope on Kitt Peak in Arizona and the Las Campanas 2.5-meter (100-inch) telescope in Chile.

The researchers found evidence for high-velocity clouds in ten of the fifteen galaxies they studied; the mass in high-velocity atomic hydrogen for the galaxies with high-velocity clouds is consistent with Galactic high-velocity clouds having a typical distance of about ten thousand light years from us. However, according to Schulman, ``Our most interesting result is that the galaxies without high-velocity clouds have significantly lower star formation rates than the galaxies with high-velocity clouds, suggesting that there is a connection between star formation and high-velocity clouds in spiral galaxies.'' Definitive proof of such a connection would go a long way towards helping astronomers understand what the high-velocity clouds are and where they came from.

One theory, proposed in 1980 by Dr. Joel Bregman (now a professor of astronomy at the University of Michigan), suggests that high-velocity clouds are created when supernovae in groups of massive stars produce expanding bubbles of hot gas with temperatures of millions of degrees. This hot gas is pushed thousands of light years above the disk of the Galaxy where it cools off and then falls back onto the Galaxy as a rain of high-velocity clouds. This theory, dubbed the ``galactic fountain model,'' predicts that about 10% of the atomic hydrogen in the Galaxy should be in the fountain at any one time, and that the process should be small or nonexistent in galaxies without sufficiently active star formation. ``Our data support both these predictions,'' said Schulman, although he was quick to point out that ``we have only studied a handful of galaxies so far.'' The researchers will be using the Very Large Array to observe two galaxies this month, and next year they intend to observe 100 galaxies or more when the new 100-meter (300-foot) Green Bank Telescope in West Virginia becomes operational.

Even if the galactic fountain model can explain most of the high-velocity clouds in the Milky Way and other galaxies, it cannot explain them all. ``There is definitely high-velocity atomic hydrogen falling onto our Galaxy from the Magellanic Clouds, two small satellite galaxies of the Milky Way,'' pointed out Schulman, ``and at least two of the galaxies in our study have some high-velocity atomic hydrogen from similar external sources.'' It is even possible that most of the high-velocity clouds are actually infalling clumps of gas that failed to be incorporated into galaxies billions of years ago. ``I would be surprised if this were the case,'' said Schulman, ``because then the mass in high-velocity atomic hydrogen should be about a factor of five smaller than we see in the galaxies we've observed so far. Also, it is unclear that a small amount of infalling material could trigger sufficient star formation to explain the higher level of star formation activity we see in the galaxies with high-velocity clouds.'' ``However,'' continued Schulman, ``it is true that the data cannot rule out that theory at this point in time.''

The team's research was supported by NASA, by the National Science Foundation's Research Experiences for Undergraduates program, and by the Carnegie Institution of Washington. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.