Matt Hall NCSA/Eric Hallman, University of Colorado

Invisible web. In this computer simulation, WHIM (blue) runs along filaments between galaxies and clusters of galaxies.

X-ray Vision Reveals Intergalactic Medium

Astronomers are claiming the detection via x-rays of a chunk of the universe's ordinary matter. Unlike the famous "dark matter" whose gravity is thought to hold the galaxies together, this more mundane form of missing matter is the stuff of normal atoms. The new observations fit well with the standard model of cosmology and could help test ideas about large-scale structures in the universe.

Three ingredients make up the universe. Normal atomic matter accounts for less than 5%, according to analysis of the microwave afterglow of the big bang. About 22% of the universe consists of mysterious dark matter--weighty stuff whose gravity appears to keep the stars from flying out of galaxies and to hold clusters of galaxies together but that has never been directly observed. A whopping 73% of the universe consists of bizarre space-stretching dark energy that's accelerating the expansion of the universe.

Even most of the ordinary matter is still missing. About half of ordinary matter has formed stars and is clearly visible. Most of the rest, which floats between galaxies, is ionized gas that is hard to see. Since 2000, astronomers have caught patches of this "warm-hot intergalactic medium" (WHIM) by, for example, spotting clouds of oxygen VI--oxygen stripped of five of its eight electrons--as they absorb the ultraviolet wavelengths in the light from quasars far behind them (ScienceNOW, 25 November 2002). But most WHIM is still unaccounted for, in large measure because the ions that trace the hotter stuff, such as oxygen VII, absorb x-rays. Because the oxygen VII is so diffuse, it is extremely difficult to detect whether x-rays have been absorbed by the gas.

Now David Buote, an astrophysicist at the University of California, Irvine, and an international team report the detection of x-rays absorbed by such missing ordinary matter. Using NASA's Chandra telescope and the European Space Agency's XMM-Newton, the team spotted this gas along a portion of the Sculptor Wall, part of a large assemblage of galaxies some 400 million light-years away. Oxygen VII between the galaxies absorbed x-rays coming from an energetic galaxy behind the Sculptor Wall. Buote gives their detection a 99.7% chance of being correct.

The team's observations, which will be published in the 20 April issue of the Astrophysical Journal, appear to be "well-planned, careful, and pretty convincing," says Michael Vogeley, an astronomer at Drexel University in Philadelphia, Pennsylvania, who was not involved in the study. "The finding that much of the regular stuff is WHIM-sical and that it is associated with the largest-scale structures seen in galaxies is a confirmation of the basic picture of how the regular stuff in the universe is predicted to behave in such a universe."

But Michael Shull, an astrophysicist at the University of Colorado, Boulder, remains cautious: "The past history of these claimed detections makes me hesitate to say this detection is certain."

Buote's team is now analyzing follow-up observations done with Chandra. In theory, much more WHIM should be detectable by x-ray telescopes. Theorists are keen to know how this matter is distributed, as that would help them test their models of how galaxies form and the universe evolved.

Posted in Space