Detecting gas clouds in cosmic voids.

Empty space isn't so empty after all. Astronomers have detected clouds of hydrogen gas in what were thought to be giant voids, hundreds of light-years across, between clusters of galaxies.

The hydrogen clouds may represent the outer, gaseous halos of galaxies too faint to show up directly on telescope images, notes study coauthor John T. Stocke of the University of Colorado, Boulder. Alternatively, the clouds may consist of pristine material, forged a few minutes after the birth of the universe, that never formed stars or coalesced into galaxies.

During the 1970s and 1980s, several teams of researchers established that galaxies in relatively nearby parts of the universe arrange themselves in thin sheets separated by giant bubbles or voids (SN: 11/25/89, p.340). Using the Hubble Space Telescope's Goddard high-resolution spectrograph, Stocke and his collaborators examined some of these seemingly empty regions of intergalactic space by analyzing the light from quasars that lie just behind the voids.

When the quasar light passes through a gas cloud, atoms in the cloud absorb some of the radiation, producing a characteristic gap in the spectra recorded by the Hubble detector. Clouds that lie at different distances along the line of sight to the quasar produce absorption gaps at different wavelengths, as seen by an observer on Earth.

Stocke and his coworkers, including Boulder astronomers J. Michael Shull and Steven V. Penton, found that two of the nine hydrogen clouds they detected lie in what appear to be empty regions of space. Extrapolating from their observations, the team estimates that low-density hydrogen clouds could collectively contain as much mass as the known population of galaxies.

But they note that most clouds do seem to avoid the voids, confirming that these regions are nearly--though not entirely--free of matter. Any theory of galaxy formation must still come to terms with this distribution of material, the researchers note.