Flying observatory seeks intergalactic gas.

Cosmologists have sought for decades to detect the intergalactic medium -- the tenuous material lying between galaxies -- and to characterize its composition. Hydrogen and helium, forged in the aftermath of the Big Bang and spread throughout the universe, presumably provided the tiny building blocks from which the first galaxies formed.

These intergalactic gases should also constitute a tiny, but key fraction of the unseen matter, or dark matter, that astronomers believe the universe harbors. Galaxies alone can't account for the density of ordinary particles, or baryons, predicted by the Big Bang, and intergalactic hydrogen and helium would contribute to the small fraction of dark matter thought to consist of baryons.

Researchers proposed in 1965 that ultraviolet light from a distant quasar could betray the presence of intergalactic hydrogen, because the gas would absorb specific wavelengths of the radiation passing through it. But observers never found evidence of atomic hydrogen or helium in the intergalactic medium, suggesting that the bulk of such gases, if present, must be ionized.

With its launch early this month aboard the space shuttle, the Astro 2 observatory will pursue a 16-day mission to view the universe with ultraviolet eyes. The observatory carries a trio of telescopes that will take images, spectra, and polarization measurements of nearby stars and distant galaxies. And it will search for the fingerprints of material that had until recently eluded detection: the tenuous mix of hydrogen and helium thought to reside outside of galaxies.

The Hopkins Ultraviolet Telescope (HUT) on Astro 2 should provide a highly accurate probe of the stuff between galaxies, report Wei Zheng and HUT team leader Arthur F. Davidsen of Johns Hopkins University in Baltimore in the Feb. 20 Astrophysical Journal Letters.

Last year, researchers reported that the Hubble Space Telescope had detected a gap, characteristic of ionized helium, in the spectrum of ultraviolet light from a quasar (SN: 7/9/94, p.21). But because of its low spectral resolution, Hubble's faint-object camera can't tell whether the absorption stems from diffuse helium in the space between galaxies or from a series of intergalactic clouds containing the ionized gas.

In their report, Zheng and Davidsen note another caveat. They say that singly ionized helium gas close to a quasar may be further ionized by the quasar light passing through it. As a result, intergalactic helium might carve a gentler absorption feature than the sharply defined gap recorded by Hubble.

HUT has several advantages over Hubble, says Kenneth M. Lanzetta of the State University of New York at Stony Brook. Because HUT detects radiation at shorter wavelengths, it can search for helium in the spectra of quasars with lower redshift. Because they would lie closer to Earth, such quasars are likely to be brighter and their light less likely to pass through hydrogen clouds. These clouds can confound results by giving a spectrum that mimics the absorption of helium at a greater distance. In addition, HUT's higher spectral resolution should distinguish intergalactic helium from helium clouds.