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University of California-Berkeley

Media Contacts:

Robert Sanders
(510) 643-6998, (510) 642-3734
rls{at}pa.urel.berkeley.edu

Additional Resources:

Barry Welsh
(510) 642-0305, bwelsh{at}ssl.berkeley.edu

FOR IMMEDIATE RELEASE: Thursday, May 29, 2003

3-D map of local interstellar space shows sun lies in middle of hole
piercing galactic plane

Part 2 of 2

"This thin shell of dense gas surrounding the local void is broken in
many places," said Dr. Francoise Crifo, an astronomer at the Paris
Observatory. "In several directions in the galaxy, our local cavity
seems to be linked with other similar empty regions by pathways or
tunnels in the interstellar medium." 

The existence of a network of tunnels of hot gas that thread
interstellar space was first suggested nearly 30 years ago by Don Cox
and Barry Smith of the University of Wisconsin. In their model,
energetic galactic supernova explosions create fast-moving expanding
bubbles of hot gas that collide with the surrounding cold gas of
interstellar space, which in turn becomes compressed into thin
shells. Eventually, these shells of cold gas meet other expanding hot
cavities and break up to form small tunnels or pathways between the
expanding voids, Welsh said.

"If you look at the new map of cold, local interstellar gas in the
galactic plane, you see an inner empty region with finger-like
extensions or tunnels that poke through the surrounding wall and
reach into nearby regions of our galaxy, such as the Lupus-Norma,
Auriga-Perseus and Scorpius-Centaurus associations of young stars,"
he said. "These regions are known to contain large amounts of hot and
ionized low-density gas that can be detected at radio and ultraviolet
wavelengths."

The new results also show that the local void of gas extends out of
the galactic disk and stretches into the overlying galactic halo
region. In this view of the galaxy, the cavity appears as a
tube-like "chimney" that links the gas in the galactic disk with that
of the halo. Galactic chimneys have been widely observed in other
galaxies and are thought to be responsible for venting the hot and
ionized gas expelled in supernova explosions into galaxy halos.
Active vents seen in other galaxies have been labeled "galactic
fountains" and the gas falling back into the disk, "galactic rain."

"We have been searching for signs of hot gas in this local chimney
using the NASA FUSE satellite, and so far, we have found tentative
signs of hot gas that seems to be coming towards us, which could be
part of a fountain that is falling back onto the galactic plane,"
Welsh said. 

The results of the research are soon to be published in the journal
Astronomy and Astrophysics, along with a 3-D map of the distribution
of cold sodium gas out to 1,000 light years. Lallement noted,
however, that the project is not yet complete.

"We also have information on the motion of the atoms of sodium gas we
have detected along the 1,005 sight-lines," she said. "Soon we will
be able to say whether the gas wall that surrounds our local void is
coming towards the sun and squeezing our local interstellar space, or
whether it is moving away from us such that the local void is getting
larger." 

"Either of these scenarios is fascinating," Welsh said. "If the wall
is approaching us, it means that a distant explosive force is pushing
it towards us. If it is expanding away from the sun, then it seems
possible that a supernova explosion took place about a million years
ago that was located relatively close to our sun."

Other colleagues are Daphne Sfeir, who published a preliminary map in
1999 using 350 stars, and J. L. Vergely of ACRI, a high-tech company
based in Sofia-Antipolis, France. The group used data from two other
telescopes -- the Kitt Peak telescope in Arizona and the recently
destroyed Mt. Stromlo telescope in Australia.

The research was supported by a National Aeronautics and Space
Administration contract with the Johns Hopkins University.

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