NASA’s Chandra X-ray Observatory has discovered part of
an intergalactic web of hot gas and dark matter that contains
most of the material in the universe. The hot gas, which
appears to lie like a fog in channels carved by rivers of
gravity, has been hidden from view since the time galaxies

“The Chandra observations, together with ultraviolet
observations, are a major advance in our understanding of how
the universe evolved over the last 10 billion years,” said
Fabrizio Nicastro, leader of Harvard-Smithsonian Center for
Astrophysics (CfA) in Cambridge, Mass., and head of one of
the teams of scientists involved in the discovery.

Four independent teams of scientists, whose results appear as
separate papers in the Astrophysical Journal, used Chandra to
detect intergalactic gas with temperatures ranging from
300,000 to 5 million degrees Celsius. This gas forms part of
a gigantic system of hot gas and dark matter that defines the
cosmic landscape. The gaseous component alone contains more
material than all the stars in the universe.

“We had strong suspicions from the Big Bang theory and
observations of the early universe that this gas exists in
the present era, but like a stealth aircraft it had eluded
our detection,” said Claude Canizares of the Massachusetts
Institute of Technology (MIT), Cambridge, who along with
Taotao Fang, also of MIT, led of one of the teams.

The hot gas detected by Chandra can be used to trace the
presence of the more massive dark-matter component. The
discovery of the hot gas may eventually enable astronomers to
map the distribution of dark matter in the universe and
perhaps understand its origin.

Ultraviolet telescopes had detected cooler components of the
hot gas system, but because of its high temperatures most of
it is detectable only with an extremely sensitive X-ray
telescope. The various groups used two techniques to probe
the intergalactic gas. One method uses the absorbing effects
of the gas on X-rays from distant galaxies.

On their way to Earth, the X-rays from a distant quasar dim
as they pass through a cloud of the intergalactic gas. By
measuring the amount of dimming due to oxygen and other
elements in the cloud, astronomers were able to estimate the
temperature, density and mass of the absorbing gas.

Observations of the quasars PKS 2155-304 by the MIT and
Harvard-Smithsonian groups, and H1821+643 by a group from
Ohio State, Columbus, revealed various parts of the hot gas
system. One of these appears to be a filament in which the
Milky Way and Andromeda galaxies are embedded, whereas other
detected portions are at distances of a few billion light-
years from Earth.

These results confirm earlier work by Joel Bregman and Jimmy
Irvin of the University of Michigan in Ann Arbor, who flipped
the normal procedure, and used the fact that the hot gas is
itself a source of X-rays. By observing the absorption of X-
rays from the hot gas by a foreground galaxy, they were able
to deduce the presence of hot gas behind the galaxy.

“Normally the doctor studies the X-ray shadow produced by
your bones to learn about your bones,” said Bregman. “In
essence, we used the shadow to learn about the X-ray

During the first few billion years of the universe, about 20
percent of the matter came together under the influence of
gravity to form groups and clusters of galaxies. Theories
predict that most of the remaining normal matter and dark
matter formed an immense filamentary web connecting the
groups and clusters of galaxies, predicted to be so hot that
it would be invisible to optical, infrared and radio

“Computer simulations have been telling us for several years
that most of the ‘missing’ gas in the universe should be in
hot filaments,” said Smita Mathur, leader of the Ohio State
team. “Most of those filaments are too faint to see, but it
looks like we are finally finding their shadows.”

NASA’s Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program, and TRW, Inc., Redondo Beach,
Calif., is the prime contractor. The Smithsonian’s Chandra X-
ray Center controls science and flight operations from
Cambridge, Mass.

Images and additional information about this result are
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