Most of the light in the universe comes from star-studded galaxies, but in terms of mass, they are just the tip of the cosmic iceberg. According to theory, the bulk of the matter in the universe consists of large, dark filaments of gas in the vast empty space between galaxy clusters. The European x-ray satellite XMM-Newton has now found new evidence for the existence of this Warm-Hot Intergalactic Medium (WHIM).
First proposed by theorists Renyue Cen and Jeremiah Ostriker of Princeton University in 1998, the WHIM is the tenuous primordial stuff from which galaxies and clusters of galaxies grew. Astronomers want to study the WHIM to better understand the evolution of the universe. Unfortunately, at a temperature of just a few million degrees (much cooler than the extremely hot gas in galaxy clusters), it is extremely hard to detect. But last summer, astronomers studying a distant quasar with NASA's Chandra X-ray Observatory claimed they found that part of the quasar's radiation was absorbed by warm-hot intervening material.
Now, thanks to the much higher sensitivity of XMM-Newton, a group led by Jelle Kaastra of the SRON National Institute for Space Research in Utrecht, the Netherlands, says it has detected the feeble x-ray emission of the WHIM. They looked in the outskirts of five galaxy clusters, where the density of the WHIM is expected to be higher than average. In particular, they detected highly ionized oxygen atoms, indicative of a temperature of a mere 1 million to 2 million degrees. "This really confirms the existence of the WHIM," says Kaastra. The results will be published in an upcoming issue of Astronomy & Astrophysics.
Some astronomers are skeptical. "To me it's unconvincing," says Stuart Bowyer of the University of California, Berkeley. However, Claude Canizares of the Massachusetts Institute of Technology in Cambridge, who led the earlier quasar study, says that the XMM-Newton results provide important new information. Although the more tenuous filaments of the WHIM haven't been seen yet, "it's an important start," he says.
To settle the issue, astronomers will probably have to wait for SPIDR (Spectroscopy and Photometry of the IGM Diffuse Radiation), a $89 million mission recently selected by NASA, says Taotao Fang of Carnegie Mellon University in Pittsburgh. SPIDR will be launched in 2005 and should "fully reveal the WHIM gas" and map its three-dimensional distribution, Fang says.