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Harvard-Smithsonian Center for Astrophysics

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For Release: February 19, 2003

Release No.: 03-06 

Missing Mass Exists As Warm Intergalactic Fog
=============================================

Cambridge, MA -- One of the fundamental questions astronomers are 
trying to answer is: What is the Universe made of?  Numerous lines of 
evidence show that the Universe is about 73 percent "dark energy," 23 
percent "dark matter," and only 4 percent normal matter. Yet this 
answer raises further questions, including: Where is all the normal 
matter? 

Astronomers call this dilemma the "missing mass" problem.  They can 
see normal, baryonic matter -- protons, electrons, and neutrons -- 
when it forms luminous stars, or when it blocks starlight as huge, 
dark molecular clouds. And what they see totals only a fraction of the 
normal matter they know is out there.

Now, astronomer Fabrizio Nicastro of the Harvard-Smithsonian Center 
for Astrophysics (CfA) and colleagues have found evidence for the 
existence of a large reservoir of baryons in our Local Group of 
galaxies. This baryonic matter forms a warm fog surrounding and 
enveloping the Milky Way and its neighbors.

"Our research shows that this warm fog may hold as much as two-thirds 
of the normal matter within the neighborhood of the Milky Way," says 
Nicastro. 

Finding The Missing Mass 
------------------------
This warm intergalactic fog is a challenge to find.  Astronomers 
cannot see it directly because it is too diffuse, despite its 
temperature of 100,000 to 10 million Kelvin (105 - 107 K), which 
causes it to shine faintly in X-rays. Instead, they detect the fog 
using the shadow it casts. Nicastro and his team looked at ultraviolet 
and X-ray wavelengths where the intergalactic fog absorbed light from 
distant sources like quasars and active galactic nuclei. They culled 
data from the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite 
to identify about 50 clouds, or fog banks, surrounding our galaxy in 
every direction.

Atoms in individual clouds absorb light at specific wavelengths, 
creating dark lines in the spectra of background light sources. The 
motion of a cloud shifts the wavelength of its spectral line due to 
the Doppler effect. Nicastro's team used these spectral shifts to
derive radial (line-of-sight) velocities for the clouds, giving clues 
to the clouds' locations and origins. Those studies showed that the 
warm clouds were almost certainly part of the Local Group of galaxies, 
which is comprised of the Milky Way and Andromeda spirals, along with 
about 30 smaller galaxies.

Given the amount of material they detected using FUSE and NASA's 
Chandra X-ray Observatory, Nicastro and his associates infer that the 
warm fog in the Local Group contains as much mass as a million million 
(10**12) Suns. This result shows remarkable agreement with the amount 
of matter needed to gravitationally bind together the galaxies within 
the Local Group. 

A Relic Of Galaxy Formation 
---------------------------
"Given the fact that this warm fog exists, it raises the question of 
where this matter came from," says Nicastro.  "Most likely, it is 
material left over from the galaxy formation process, a relic from the 
early history of the Universe."

Theories indicate that the early Universe was filled with a nearly 
homogeneous mix of hydrogen and helium gas.  Clumps of dark matter 
within this primordial soup acted as seeds for galaxy formation. Over 
several hundred million years of time, the force of gravity pulled
together some of the Universe's normal matter to form galaxies holding 
billions of stars. 

However, only about one-third of the Universe's baryonic matter was 
consumed. Much of it still floats between the galaxies, invisible 
except for the shadow it casts. 

"Finding this leftover material provides further evidence that our 
theories of galaxy formation are correct and offers clues to the 
history of our own Milky Way galaxy," says Nicastro. "This discovery, 
combined with future research, also may help track dark matter because 
the intergalactic filaments of baryonic matter should connect the dark 
matter clumps." 

This research was reported in the February 12, 2003, issue of the 
scientific journal Nature in a paper authored by Fabrizio Nicastro 
(CfA); Andreas Zezas and Martin Elvis (CfA); Smita Mathur (Ohio State
University); Fabrizio Fiore (Osservatorio Astronomico di Monteporzio); 
Cesare Cecchi-Pestellini, Douglas Burke, Jeremy Drake, and Piergiorgio 
Casella (CfA). 

Headquartered in Cambridge, Massachusetts, the Harvard- Smithsonian 
Center for Astrophysics (CfA) is a joint collaboration between the 
Smithsonian Astrophysical Observatory and the Harvard College 
Observatory. CfA scientists organized into six research divisions 
study the origin, evolution, and ultimate fate of the universe.

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