Press Release

Scientists Announce Discovery of New Class of Galaxy

By SpaceRef Editor
May 30, 2003
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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

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

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

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:

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
Energyís National Nuclear Security Administration.

SpaceRef staff editor.