"Missing Mass" Found in Recycled Dwarf Galaxies (Forwarded)

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EMBARGOED For Release: 2:00 p.m., EDT, Thursday, May 10, 2007

"Missing Mass" Found in Recycled Dwarf Galaxies

Astronomers studying dwarf galaxies formed from the debris of a collision
of larger galaxies found the dwarfs much more massive than expected, and
think the additional material is "missing mass" that theorists said should
not be present in this kind of dwarf galaxy.

The scientists used the National Science Foundation's Very Large Array
(VLA) radio telescope to study a galaxy called NGC 5291, 200 million
light-years from Earth. This galaxy collided with another 360 million
years ago, and the collision shot streams of gas and stars outward. Later,
the dwarf galaxies formed from the ejected debris.

"Our detailed studies of three 'recycled' dwarf galaxies in this system
showed that the dwarfs have twice as much unseen matter as visible matter.
This was surprising, because they were expected to have very little unseen
matter," said Frederic Bournaud, of the French astrophysics laboratory AIM
of the French CEA and CNRS. Bournaud and his colleagues announced their
discovery in the May 10 online issue of the journal Science.

"Dark matter," which astronomers can detect only by its gravitational
effects, comes, they believe, in two basic forms. One form is the familiar
kind of matter seen in stars, planets, and humans -- called baryonic
matter -- that does not emit much light or other type of radiation. The
other form, called non-baryonic dark matter, comprises nearly a third of
the Universe but its nature is unknown.

The visible portion of spiral galaxies, like our own Milky Way, lies
mostly in a flattened disk, usually with a bulge in the center. This
visible portion, however, is surrounded by a much larger halo of dark
matter. When spiral galaxies collide, the material expelled outward by the
interaction comes from the galaxies' disks. For this reason, astronomers
did not expect that "recycled" dwarf galaxies formed from this collision
debris would contain much, if any, dark matter.

When Bournaud and his international team of scientists used the VLA to
study three dwarf galaxies formed from the debris of NGC 5291's collision,
they were surprised to find two to three times the amount of dark matter
as visible matter in the dwarfs. They determined the dwarfs' masses by
measuring the Doppler shift of radio waves emitted by atomic Hydrogen at a
frequency of 1420 MHz. The amount of shift in the frequency indicated the
rotational speed in the galaxy. That, in turn, allowed the scientists to
calculate the dwarf's mass.

Images from two NASA satellites provided vital information about the dwarf
galaxies. "Using ultraviolet images from the Galex satellite and infrared
data collected by the Spitzer satellite, we had previously shown that the
dwarfs all along the debris stream were star-forming galaxies," said
Pierre-Alain Duc, also of the AIM laboratory (CEA/CNRS).

What is the dark matter in the dwarfs? The astronomers don't believe it is
the mysterious non-baryonic type, but rather cold Hydrogen molecules that
are extremely difficult to detect.

When the astronomers performed computer models of the collision of NGC
5291 to simulate the formation of the system seen today, the models left
the resulting recycled dwarfs with almost no dark matter. These computer
models had started off with all the dark matter in the galaxy's larger
halo.

"The result of the computer models means that the additional mass we see
in the real dwarfs came from the disks, not the haloes, of the larger
galaxies that collided," Bournaud said. That additional mass, the
scientists believe, almost certainly is "normal" baryonic matter, probably
cold molecular Hydrogen.

While the discovery about NGC 5291's neighboring dwarf galaxies sheds new
light on the composition of spiral galaxies, it doesn't tell the
scientists anything about the non-baryonic dark matter, whose nature
remains a mystery. "Still, this new information about the matter
comprising galactic disks should help us work toward a better
understanding of their formation and evolution," Bournaud concluded.

Bournaud and Duc worked with Mederic Boquien, also of the AIM laboratory
(CEA/CNRS); Elias Brinks of the University of Hertfordshire in the UK;
Phillipe Amram of the Astronomical Observatory of Marseille-Provence; Ute
Lisenfeld of the University of Granada, Spain; Barbel S. Koribalski of the
Australia Telescope National Facility; Fabian Walter of the Max Planck
Institute for Astronomy in Heidelberg, Germany; and Vassilis Charmandaris
of the University of Crete, Greece.

The National Radio Astronomy Observatory is a facility of the National
Science Foundation, operated under cooperative agreement by Associated
Universities, Inc. The California Institute of Technology leads the Galaxy
Evolution Explorer mission and is responsible for science operations and
data analysis. NASA's Jet Propulsion Laboratory, a division of Caltech,
manages the mission and built the science instrument, and also manages the
Spitzer Space Telescope.

[NOTE: Images supporting this release are available at
http://www.nrao.edu/pr/2007/darkdwarfs/graphics.shtml ]