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1996-01-12
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FOR RELEASE: March 31, 1995
CONTACT: Don Savage/NASA Headquarters
202-358-1547
Tammy Jones/Goddard Space Flight Center
301-286-5566
Ray Villard/Space Telescope Science Institute
410-338-4514
PRESS RELEASE NO.: STScI-PR95-22
HUBBLE DATA SUGGEST GALAXIES HAVE GIANT HALOS
NASA's Hubble Space Telescope has helped solve a two-decade old cosmic
mystery by showing that mysterious clouds of hydrogen in space may
actually be vast halos of gas surrounding galaxies.
"This conclusion runs contrary to the longstanding belief that these
clouds occur in intergalactic space," says Ken Lanzetta of the State
University of New York at Stony Brook.
The existence of such vast halos, which extend 20 times farther than
the diameter of a galaxy, might provide new insights into the evolution
of galaxies and the nature of dark matter - an apparently invisible
form of matter that surrounds galaxies.
The possibility of galaxy halos was first proposed in 1969 by John
Bahcall and Lyman Spitzer of the Institute for Advanced Study,
Princeton, NJ. Previous observations with ground-based telescopes,
the International Ultraviolet Explorer satellite, and Hubble have
suggested that these clouds might be galaxy halos. However, the
latest results are the most definitive finding yet, says Lanzetta,
because they come from a large sample of 46 galaxies.
For the past two decades, observations with ground-based telescopes
have shown that the light from distant quasars (the bright cores of
active galaxies) is affected by intervening gas clouds. These clouds
are invisible, but betray their presence by absorbing certain
frequencies, or colors, of a quasar's light. When a quasar's light is
spread out into a spectrum, the missing wavelengths appear as a complex
"thicket" of absorption features. Ground-based observations also
showed that the number of these clouds rapidly rises out to greater
distance. One possible explanation was that these were primordial
clumps of gas that dissipated over time.
However, in 1991, independent observations made with Hubble's Faint
Object Spectrograph and Goddard High Resolution Spectrograph
instruments detected more than a dozen hydrogen clouds within less than
a billion light-years of our galaxy. These clouds could not be
detected previously because they are only visible in the ultraviolet
part of the spectrum, which is inaccessible with ground-based
telescopes. This gave astronomers a powerful opportunity to further
test the halo theory by imaging nearby galaxies and attempting to match
them with nearby clouds.
Lanzetta, David Bowen of the Space Telescope Science Institute (STScI),
Baltimore, MD, David Tyler of the University of California at San
Diego, and John Webb of the University on New South Wales, Australia,
attempted to match galaxies and clouds by first collecting Hubble
archival data on six quasars. Next, using telescopes at the National
Optical Astronomy Observatory, the Anglo Australian Observatory, the
Lick Observatory and the Isaac Newton Telescope, they identified
galaxies near the clouds and measured distances. In the majority of
cases they found galaxies within about 500,000 light-years of the
clouds.
"These results are a surprise. We have never seen these halos in the
local universe," said David Bowen of STScI.
The results explain why so many clouds are seen at greater distances:
the light from distant quasars was more likely to pass through a
galaxy's halo because the halo is so large.
These results appear in the April 1 issue of the Astrophysical
Journal. The researchers plan to extend their research to a larger
sample of galaxy/cloud pairs.
* * * *
The Space Telescope Science Institute is operated by AURA (the
Association of Universities for Research in Astronomy, Inc.) for NASA,
under contract with the Goddard Space Flight Center, Greenbelt, MD.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (the European Space Agency).