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Nearby gas clouds pose cosmological puzzle.

Astronomers have for the first time counted the hydrogen clouds neighboring our galaxy. Their analysis of quasar light, using detectors on the Hubble Space Telescope, reveals many more clouds than predicted. Because some hydrogen clouds represent material that has failed to coalesce into galaxies, the surprising abundance of nearby gaseous bodies may have far-reaching consequences for the structural development of the universe, researchers say.

Hydrogen clouds have been called the Rosetta stones of the universe, the key to understanding its history. Some clouds may date to the Big Bang, and some provide the raw material for galaxies. Though they offer clues to the evolution of the cosmos, these clouds emit little light and have never been seen. Instead they make their presence known by absorbing specific wavelengths as quasar light passes through on its way to Earth.

As observed near Earth, each cloud absorbs a different wavelength of light, depending on the cloud's location. Due to the expansion of the universe, clouds farther from Earth move faster and appear to absorb redder light wavelengths, which are detectable from the ground. The more slowly moving near-Earth clouds absorb a spectrum of ultraviolet light that can only be observed from space. Together, the clouds create a thicket of absorption lines -- called the Lyman alpha forest -- within the spectra of quasars.

Ground-based instruments measure absorption spectra from only the most distant clouds, looking back to a time when the universe was just 10 to 20 percent of its current age. Those limited observations, made during the past decade, showed that the high density of clouds creating the Lyman alpha forest began to thin rapidly with decreasing distance from the Milky Way. Researchers reasoned that if the thinning continued, regions near our galaxy should contain one or two clouds at most. Now, two Hubble instruments tell a different story.

Last week, at a science writers' workshop in Baltimore, Ray J. Weymann of the Carnegie Observatories, based in Pasadena, Calif., announced his team had found nine to 16 hydrogen clouds relatively nearby -- 30 million to 1.6 billion light-years from our galaxy. They based their findings on the absorption of light emitted by the quasar 3C273, as determined by Hubble's Goddard High-Resolution Spectrograph.

This week, John N. Bahcall of the Institute for Advanced Study in Princeton, N.J., provided SCIENCE NEWS with new details about the detection of clouds using light from the same quasar, but analyzed by Hubble's Faint-Object Spectrograph. He says that five of the seven clouds found by his team and announced last month (SN:5/4/91, p. 285) are among those detected by Weymann's group. These include two clouds that lie a mere 30 million light-years from the Milky Way and appear associated with the Virgo cluster of galaxies.

The new findings alone won't prompt a revision of galaxy formation theories, Bahcall notes. "We've first go to further characterize the darn things [clouds]," he says -- determining, for instance, whether they formed during the Big Bang or much more recently.

Using Hubble, Bahcall plans to search for metal elements in the clouds, which would indicate the clouds have formed some stars and may act as budding galaxies surprisingly late in the history of the universe. Other puzzles remain, he says, such as whether these clouds are gravitationally bound to galaxies. Their association with galaxies could account for the clouds' survival, possibly over billions of years, Bahcall explains. But if instead the clouds are independent, then extra hidden mass -- perhaps a theorized material called cold dark matter (SN:1/26/91, p. 52) -- may supply the gravity to confine each as separate entities, he adds.
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Title Annotation:existence of hydrogen clouds near our galaxy and galaxy formation theories
Author:Cowen, Ron
Publication:Science News
Date:May 25, 1991
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