Detecting helium in the early universe.

Astronomers have long recognized that the space between stars is filled with a thin mixture of gas and dust. But evidence of whether a tenuous gas also pervades the vast expanses between galaxies has remained elusive.

Now, researchers have obtained what may be the first glimpse of this diffuse intergalactic medium. Peter Jakobsen of the European Space Agency in Noordwijk, the Netherlands, and his collaborators report detecting ionized helium along the line of sight to a distant quasar.

"This breakthrough for cosmology provides the first direct evidence that the intergalactic medium is a highly ionized plasma of hydrogen and helium," comments Kenneth M. Lanzetta of the State University of New York at Stony Brook.

The discovery also confirms the existence of substantial amounts of helium in the early universe, as predicted by theories of the formation of chemical elements shortly after the Big Bang, the researchers say.

Jakobsen and his colleagues report their findings in the July 7 NATURE.

The researchers used the Faint Object Camera aboard the refurbished Hubble Space Telescope to look at ultraviolet light coming from a quasar. Any helium ions with a single positive charge along the line of sight to the quasar would absorb light of a characteristic wave-length, leaving a gap in the spectrum of light detected by the telescope.

To make the scheme work, Jakobsen and his coworkers had to find a quasar that was unobscured by massive hydrogen clouds along the path, which would cut off the helium ions' signal.

The quasar also had to be distant. Singly ionized helium normally absorbs light at 30.4 nanometers, which is well beyond the range detected by the camera. But for a sufficiently distant source, the shift to longer wavelengths caused by the universe's expansion brings this light within range.

"We're confined to looking at very remote quasars with redshifts above 3," Jakobsen says. "And we have to find a break in the [hydrogen] cloud cover at this high redshift."

The researchers came up with one candidate -- the quasar designated Q0302-003. To their delight, they found that the spectrum of uitraviolet light from this object showed strong absorption at wavelengths that could be attributed to helium ions.

"We're very happy the helium is there," Jakobsen says. The fact that this helium exists as singly charged ions rather than neutral atoms also confirms notions that the early universe was a harsh environment for atoms, he adds.

"The big surprise, however, is the intensity of the absorption," Jakobsen says. Such a large effect is hard to explain on the basis of helium-containing clouds alone.

"The obvious guess is that there's helium between the clouds," he says. "There's a significant chance we're seeing the intergalactic medium."

To determine the proportion of ionized helium in clouds and in the intergalactic medium, the researchers must obtain a spectrum with enough resolution to reveal the characteristic absorption patterns of individual clouds. This requires an exposure lasting tens of hours with one of the telescope's present spectrographs or a wait of several years until an improved spectrograph is delivered to the telescope in the next servicing mission.

"We need to observe it longer," Jakobsen says. "And we have to look for other possible candidates."