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Found: Part of the Universe’s missing matter

Posted: May 7, 2008

Using ESA’s orbiting X-ray observatory XMM-Newton, a team of international astronomers has uncovered part of the missing matter of the Universe, in a filament of gas connecting two galaxy clusters.

The composition of the Universe puzzles astronomers: Over 90 percent consists of unknown matter, of which three-quarters is made up of dark energy, which causes an accelerated expansion of the Universe, and around one-fifth is dark matter. Just four percent is composed of ‘normal’ matter, that is, protons and neutrons, or baryons, and electrons. And of that, a large portion is unaccounted for: all the stars, galaxies and gas observable in the Universe constitute less than half of all the baryons that should be present. For around ten years astronomers have imagined the missing matter as a hot, ultra thin gas that lies between galaxies, but its low density nature has hampered many previous attempts to identify it. Now, with XMM’s high sensitivity, astronomers have detected its hottest parts bridging the gap between two galaxy clusters: Abell 222 and Abell 223, which are located 2300 million light-years from Earth.

A bridge of hot X-ray emitting gas connects the two galaxy clusters Abell 222 and Abell 223. Image: ESA/ MM-Newton/EPIC/ES/ SRON/MPE.

"The hot gas that we see in this bridge or filament is probably the hottest and densest part of the diffuse gas in the cosmic web, believed to constitute about half the baryonic matter in the universe," says Norbert Werner from SRON Netherlands Institute for Space Research. “The discovery of the warmest of the missing baryons is important,” adds Alexis Finoguenov from the Max Planck Institute for Extraterrestrial Physics (MPE). “That’s because various models exist and they all predict that the missing baryons are some form of warm gas, but the models tend to disagree about the extremes.”

Even with XMM-Newton’s sensitivity, the discovery was only possible because the filament is along the line of sight, concentrating the emission from the entire filament in a small region of the sky. "So far we could only see the clusters, the dense knots of the cosmic web,” says astrophysicist Aurora Simionescu, also from MPE. “Now we are starting to see the connecting wires of the immense cosmic spider web.”

The discovery of the gas is a significant step forward on the way to a complete understanding of the so-called cosmic web, which is used to describe the distribution of matter in the Universe, and what happened after the big bang and therefore which forces are dominating the Universe today. And now that the astronomers know where to look it is likely that many follow-up studies with XMM-Newton will target these highly promising regions of the sky, and it is hoped, say the XMM astronomers, that a much higher sensitivity observatory will be launched in the future to continue the search.