Dolores Beasley
Headquarters, Washington        July 31, 2002
(Phone: 202/358-1753)

Steve Roy
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256/544-6535)

Megan Watzke
Chandra X-ray Observatory Center, CfA, Cambridge, Mass.
(Phone: 617/496-7998)

RELEASE: 02-145

CHANDRA DISCOVERS "RIVERS OF GRAVITY" THAT DEFINE COSMIC 
LANDSCAPE

     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 
formed.

"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 
machine."

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 
telescopes. 

"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 
available at:
http://chandra.harvard.edu
and
http://chandra.nasa.gov

                      -end-