The heavens may be strewn with stars, galaxies, and nebulae, but the fact is astronomers don't know precisely where most of the ordinary matter in the universe is hiding. A new x-ray observation could help untangle that mystery: Astronomers have located a filament of hot gas stretching all the way from one cluster of galaxies to another. The filament is thought to be one thread in a vast web containing the missing ordinary matter, and, if confirmed, it could give scientists a better idea of where the rest of the stuff is lurking.
To borrow a phrase from former U.S. Defense Secretary Donald Rumsfeld, the universe is chock-full of "known unknowns." Exquisitely precise measurements of the lingering afterglow of the big bang and other studies show that the cosmos consists of 74% "dark energy," weird stuff that stretches space; 22% "dark matter," known only by its gravity; and just 4% ordinary matter, such as atoms, stars, planets, and people. Scientists don't know what dark energy and dark matter are, and even half of the ordinary matter eludes detection.
That's because after the first stars and galaxies began to shine about 13 billion years ago, their light started to ionize the gas in intergalactic space. The ionized gas cannot absorb intervening light, making it hard to detect across the cosmos. But knowing exactly how such ordinary matter is distributed would help cosmologists test their models of how the universe took shape.
Now a team led by astrophysicist Norbert Werner of the Netherlands Institute for Space Research (NISR) in Utrecht has spied a portion of the missing matter in a different way, by looking at the faint x-rays it emits. The astronomers spotted a filament of hot gas stretching between two clusters of galaxies, located about 50 million light-years apart and 2.3 billion light-years away. The clusters, called Abell 222 and Abell 223, interact with and heat the gas in the intergalactic space between them--gas that vastly outweighs the matter in the clusters' galaxies. The hot gas then glows in x-rays.
Three years ago other researchers uncovered hints that the twin Abell clusters might contain some of the missing matter, but the observations were inconclusive. So a team from the Netherlands employed a sensitive instrument aboard the European Space Agency's XMM-Newton spacecraft. Today in Astronomy & Astrophysics, the researchers report new observations they say clinch the case. "We believe that we see most of the gas in this filament," Werner says. "The temperature and the density of the gas indicate that it comes from the missing matter."
Astrophysicist and co-author Jelle Kaastra, also from NISR, adds that it might be difficult to tell how much more of the gas occurs in this form, "because models predict that there is a large variety of properties of the gas corresponding to the missing matter."
Werner's group has detected a component of the missing matter that is "much hotter and denser than the more pervasive and diffuse component," says astrophysicist Smita Mathur of Ohio State University in Columbus. "We expected such [x-ray] emissions from dense regions," she says, but "this is perhaps the first time we actually saw them." One question about the discovery is whether the location of the filament between the clusters somehow has created a special case, says astrophysicist Craig Sarazin of the University of Virginia, Charlottesville. "If it's a general result," he says, "then it's very important."