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 long-standing 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
distances. 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,
Baltimore, MD, David Tyler of the University of California at San
Diego, and John Webb of the University of 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. 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.
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 the European Space Agency.
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- T o d d E. V a n H o o s e a r -
``'''vanhoose@lalaland.cl.msu.edu - vanhoose@msu.edu - vanhoose@lalaland.cl.msu.edu
(._.) Michigan State University - East Lansing, MI USA
(_) Computer Laboratory - Department of Communication
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