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
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