Recent discoveries made with CSIRO’s Parkes telescope appear to have
settled a 40-year-old controversy about the nature of gas clouds that
surround our Milky Way Galaxy.

Small ‘high-velocity’ clouds of hydrogen gas seen outside our Galaxy are
mostly scraps shed by satellite galaxies orbiting the Milky Way,
researchers say — a kind of cosmic dandruff. They are not the missing
clumps of dark matter predicted by the ‘cold dark matter’ theory of
galaxy formation.

Dr Mary Putman of the University of Colorado will present this
conclusion today [Tuesday 6 May] at a meeting about our Galaxy and its
neighbours at the Space Telescope Science Institute in Baltimore MD.

Dr Putman and colleagues in Australia used the CSIRO Parkes 64-m radio
telescope in eastern Australia to make the most detailed study to date
of the Magellanic Stream — a ribbon of hydrogen gas trailing from the
Milky Way’s largest satellite galaxies, the Large and Small Magellanic
Clouds. They also studied the high-velocity clouds that surround the Stream.

The high-velocity clouds were discovered 40 years ago, but it has been
very difficult to determine exactly how far away and massive they are.
Their nature and origin has been controversial.

Some of the high-velocity clouds are associated with the Magellanic
Stream, the researchers found.

Others appear to be shed by other satellite galaxies.

“We now have pretty conclusive evidence that the high-velocity clouds
are not scattered throughout our Local Group of galaxies, but are within
the extended halo of the Milky Way,” said Dr Putman.

“Previous studies of other galaxy groups haven’t found any counterparts
of the high-velocity clouds,” said team member Dr Lister Staveley-Smith
of the CSIRO Australia Telescope National Facility.

“If the clouds are there, they must lie close to the big galaxies rather
than be scattered throughout galaxy groups.”

The standard model of galaxy formation predicts that there are hundreds
of small lumps or ‘halos’ of cold dark matter scattered throughout the
Local Group.

But researchers have failed to find them.

It had been suggested that the high-velocity clouds might be those
objects. But the new finding means this is highly unlikely.

“If the dark matter halos the models predict are there, they can’t be
associated with large amounts of cold gas,” Dr Putman said. “We don’t
find the gas clouds at large distances from galaxies.”

“High-velocity clouds are fantastic probes of the properties of the
Milky Way and its halo,” Dr Staveley-Smith said.

“But the question of whether they individually contain much dark matter
remains controversial.”

The members of the research team were: Mary E. Putman (Center for
Astrophysics and Space Astronomy, University of Colorado), Lister
Staveley-Smith (CSIRO Australia Telescope National Facility), Kenneth C.
Freeman (Research School of Astronomy and Astrophysics, Australian
National University), and Brad K. Gibson and David G. Barnes (both
Centre for Astrophysics and Supercomputing, Swinburne University of
Technology). The Parkes telescope is operated by CSIRO Australia
Telescope National Facility as a national facility.

Images:
http://www.atnf.csiro.au/news/press/images/magellanic_pics/

More information:

Helen Sim
CSIRO Australia Telescope National Facility
+61-2-9372-4251, +61-419-635-905
Email: Helen.Sim@csiro.au

Dr Mary Putman
Center for Astrophysics and Space Astronomy
University of Colorado
+1-303-819-2067 (mobile)
During May 5-8: +1-410-338-497 (Space Telescope Science Institute meeting)
and
+1-410-235-5400 (“Doubletree Inn at the Colonnade” hotel)
+1-303-492-6058 (Colorado office)
Email: Mary.Putman@colorado.edu

Dr Lister Staveley-Smith
CSIRO Australia Telescope National Facility (Sydney, Australia)
+61-2-9372-4271
Email: Lister.Staveley-Smith@csiro.au

Dr Ken Freeman
Research School of Astronomy and Astrophysics
Australian National University
+61-2-6125-0264, +1-410-338-4970
During May 5-8: (Space Telescope Science Institute meeting)
Email: kcf@mso.anu.edu.au

Prof. Brad Gibson
Swinburne University of Technology (Melbourne, Australia)
+61-3-9214-8036
Email: bgibson@swin.edu.au

Meeting

The Space Telescope Science Institute Spring Symposium
“The Local Group as an Astrophysical Laboratory”
May 5 – 8, 2003
http://sd.stsci.edu/Astrophysical_Laboratory/index.html

Publication details: Mary E. Putman, Lister Staveley-Smith, Kenneth C.
Freeman, Brad K. Gibson and David G. Barnes. “The Magellanic Stream,
High-Velocity Clouds and the Sculptor Group,” ApJ, V. 586, p. 170-194,
2003 March 20

The paper can also be downloaded from:
http://xxx.lanl.gov/abs/astro-ph/0209127