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Lawrence Livermore National Laboratory

Contact: Anne Stark
Phone: (925) 422-9799
E-mail:stark8{at}llnl.gov

EMBARGOED 10 a.m. PDT May 28, 2003

NR-03-05-07

Scientists Announce Discovery of New Class of Galaxy

LIVERMORE, Calif. -- A major international research team has
discovered a new type of galaxy, which they have dubbed
"ultra-compact dwarf galaxies" (UCDs). The galaxies are so compact in
appearance that they have previously been misclassified as nearby
stars, causing them to be overlooked by other galaxy surveys.

A team of eight astrophysicists from the United States, Australia,
Germany, and the United Kingdom made the finding as reported in the
May 29 edition of Nature. 

Project co-leaders, research astronomer Michael Gregg of the
University of California at Davis and the Lawrence Livermore National
Laboratory and Michael Drinkwater of the University of Queensland in
Australia said the discovery confirms a suspicion held for years by
some astronomers. 

"There has been speculation for 25 years that existing galaxy surveys
have completely missed some types of galaxies, for instance, very
diffuse or very compact galaxies," Gregg and Drinkwater said.

A normal, large galaxy like our own Milky Way is about 100,000 light
years across and contains 100 billion stars, while a typical dwarf
galaxy is 10 to 100 times smaller in both size and number of stars.
The newly recognized UCDs squeeze their stars into a region only
1/500 the diameter of the Milky Way, making them very compact objects
and hard to distinguish from single, nearby stars in a photograph.

The researchers found the ultra-compact dwarf galaxies while
observing the Fornax galaxy cluster, which contains about 300 known
galaxy members that sit about 60 million light years distant from
Earth. 

"Fornax is one of the closest galaxy clusters, yet it is difficult to
tell whether a galaxy that appears small is a tiny member of the
cluster or is a giant galaxy that lies in the same direction but is
much farther away," Gregg said. 

"Our Fornax Cluster Survey used new instruments to measure the
distances to about 14,000 objects in the direction of the cluster,
enabling us to separate cluster members from background galaxies and
foreground stars," Drinkwater said. 

The discovery was made using the Anglo-Australian Telescope (AAT) at 
Coonabarabran, and the objects were investigated further using the
Hubble Space Telescope.

The exploratory survey work was facilitated by using an AAT
instrument known as the Two Degree Field Spectrograph, which can
observe up to 400 targets simultaneously. In the patch of sky that
includes the Fornax cluster, the group has now measured more than
3,500 objects, of which 1,000 are galaxies, well behind the Fornax
cluster. Of the remaining 2,500 objects, expected to be ordinary
nearby stars, seven turned out to be the ultra-compact dwarfs, a
completely new class of galaxy.

The researchers won highly valued time on the Hubble Space Telescope
to measure precisely how big these dwarf galaxies are. They then used
the powerful European Southern Observatory's Very Large Telescope
(VLT) in Chile and the University of California's Keck Telescope in
Hawaii to measure how fast stars are orbiting around within each of
the newly cataloged UCD galaxies. 

The measurements of size and star speeds can be combined to "weigh"
the galaxies to find out how massive they are, and confirm their
classification as distinct from other known types of galaxy.

Gregg and Drinkwater believe that the new galaxies will prove
important in testing theories of how galaxies in densely populated
regions like the Fornax Cluster are transformed and even destroyed
over time by gravitational effects. For instance, they say it is
possible that the UCDs are the surviving dense nuclei of larger
objects, which have been whittled away over the eons by repeated
close encounters with several giant galaxies in the cluster. To test
this idea, the research team is now pursuing additional observations
of the Fornax UCDs and has begun work to find similar objects in
other clusters of galaxies.

Other team members include: Michael Hilker of Bonn University; Kenji
Bekki and Warrick Couch of the University of New South Wales; Harry
Ferguson of the Space Telescope Science Institute in Baltimore; Bryn
Jones of the University of Nottingham; and Steven Philipps of the
University of Bristol. 

For images, go to:
http://www.llnl.gov/llnl/06news/NewsMedia/UCD_galaxies_images.html

Founded in 1952, Lawrence Livermore National Laboratory is a national
security laboratory, with a mission to ensure national security and
apply science and technology to the important issues of our time.
Lawrence Livermore National Laboratory is managed by the University
of California for the U.S. Department of Energy’s National Nuclear
Security Administration.

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