Galactic X-ray Glow Confirms Evidence of Missing Matter

The spectral glow of an oxygen isotope from three clusters of galaxies might be proof that hot gases there account for a large fraction of the previously unseen matter in the universe.

Using the European Space Agency's XMM/Newton Observatory, a team led by Dr. Jelle Kaastra at the Space Research Organization of the Netherlands identified warm gas around the centers of three massive cosmological structures as oxygen VII, giving a relatively precise reading of the temperatures found in those regions.

Results of this research will be published in Astronomy & Astrophysics (The on-line version is at:

Related research was done by Dr. Jukka Nevalainen at the Harvard-Smithsonian Center for Astrophysics and Dr. Massimiliano Bonamente at the University of Alabama in Huntsville (UAH).

Their papers will be published in the Astrophysical Journal (The on-line versions are available at, and at

These findings by Kaastra, Nevalainen and Bonamente might confirm earlier research by UAH's Dr. Richard Lieu and colleagues in California and England. In 1995, using NASA's Extreme Ultraviolet Imager, they found a faint ultraviolet and X-ray glow coming from the hearts of several clusters of galaxies.

They theorized that the glow came from gaseous clouds filling billions of cubic light years -- enough gas to represent a significant fraction of the missing mass of the universe.

Clusters of galaxies are the largest cosmological structures in the universe. Each galaxy contains several tens of billions of stars, while each galaxy cluster includes hundreds or thousands of galaxies spanning millions of light years.

These massive clusters have so much mass (and so much gravity) that each cluster traps at its center a vast volume of gas that is heated to several tens of millions of degrees, with a cumulative mass equal to one hundred trillion suns.

That's where one of the mysteries begins. To have enough gravity to hold that much gas in place, a cluster needs a mass equal to about a quadrillion suns. With only a few tens of trillions of stars twinkling in its constituent galaxies, that left the clusters with a lot of missing (or at least undetected) stuff floating around.

Another mystery involved the evolution of galaxy clusters. Among the competing theories is one proposing that as clusters grew, they pulled random atoms floating through intergalactic space into seed regions in the cluster centers. As they fell these atoms would gradually be heated, eventually to X-ray emitting temperatures.

"Our earlier reports of an extreme ultraviolet and soft X-ray glow from clusters provided the first clues that the theory proposing warm gas still in the process of being heated was alive and kicking," said Lieu.

What was missing, however, was direct observational evidence of a gas supply outside of the clusters. If the theory is correct, there should be a glow of radiation that is cooler as it goes away from the center of each cluster.

The oxygen VII spectral signature found in the outlying gas of three clusters provides a sensitive thermometer to measure the temperature of gas that is emitting "soft" X-rays. At low temperatures oxygen atoms are neutral and oxygen VII doesn't exist. Any hotter and the atoms' electrons are stripped away.

The narrow temperature range in which oxygen VII exists places the temperature of the outlying gas clouds at levels cooler than those of gas at the core of each cluster.

"These oxygen emission lines provide clinching evidence of a vast amount of warm intergalactic matter lying immediately outside clusters," said Lieu. "The intensity of this emission also tells us how much mass the gas has."

The mass of gas inferred from the oxygen emissions is as much as 50 percent of the total expected mass of gas, stars and other known forms of matter in a galaxy cluster, said Lieu. These estimates indicate that the ambient universe does contain enough previously undetected matter to account for the formation of giant structures, including galaxy clusters, through the gravitational infall of gas into central "seed" regions.

For additional information:

  • Dr. Jelle Kaastra, 31-30-253-8570 Space Research Organization of the Netherlands
  • Dr. Jukka Nevalainen, (617) 496-2098 Harvard-Smithsonian Center for Astrophysics
  • Dr. Richard Lieu, (256) 824-2859 The University of Alabama in Huntsville
  • Dr. Massimiliano Bonamente, (256) 824-1630 The University of Alabama in Huntsville

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