X-ray glow from clusters of galaxies 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: http://xxx.lanl.gov/abs/astro-ph/0210684).

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 http://xxx.lanl.gov/abs/astro-ph/0210610, and
at http://xxx.lanl.gov/abs/astro-ph/0211439).

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