Hidden galaxies

Members of a newly discovered class of galaxies are so small that astronomers had mistaken them for foreground stars.
By | Published: June 4, 2003
Ultra-compact Dwarf Galaxies
Hubble Space Telescope images of five of the newly identified UCDs and a normal dwarf galaxy (dEN) are shown along with their positions in the Fornax galaxy cluster. The positions of the two UCDs not pictured are also shown.
Michael Hilker; Background: Arna Karick (Univ. of Melbourne), Curtis Schmidt Telescope; Insets: HST
June 4, 2003
An in-depth study of the nearby Fornax galaxy cluster revealed that some objects previously believed to be foreground stars belong to a new class of galaxy designated “ultra-compact dwarf galaxies” or UCDs. Members of this newly discovered type of galaxy compress their stars into a region so small each system is only two to twenty percent the size of a typical dwarf galaxy. UCDs are 500 times smaller than a normal-sized galaxy, such as the Milky Way with its 100 billion stars spanning about 100,000 light-years.

Astronomers had long thought that existing galaxy surveys were missing special galaxy types, such as very diffuse or very compact galaxies. “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,” says Michael Gregg, one of the project’s co-leaders from the University of California at Davis and the Lawrence Livermore National Laboratory. The team of astrophysicists published their findings of UCDs in the May 29 edition of Nature.

The scientists used the Anglo-Australian Telescope, Hubble Space Telescope, Very Large Telescope, and Keck Telescope to detect and measure these dense new galaxies. “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,” says Michael Drinkwater, the team’s other co-leader from the University of Queensland in Australia.

Ultra-compact Dwarf Galaxy
The two insets are Hubble Space Telescope images of a normal dwarf galaxy (upper left) and a recently discovered ultra-compact dwarf galaxy (lower right). The object running diagonally in the middle is a star trail that would result from a tidal disruption of the halo of a normal dwarf galaxy, producing a compact dwarf galaxy. The background shows the Fornax galaxy cluster, where seven ultra-compact dwarf galaxies have been found.
Background: Arna Karick (University of Melbourne), Michigan Curtis Schmidt Telescope; Insets: HST
An instrument known as the Two Degree Field Spectrograph on the Anglo-Australian Telescope targeted the Fornax cluster, which lies about 60 million light-years from Earth and contains 300 known galaxy members. The spectrograph, which can look at up to 400 targets simultaneously, measured 3,500 objects for the study. The scientists found that 1,000 of those are background galaxies. Of the remaining 2,500 objects that were believed to be regular foreground stars, seven actually turned out to be UCDs.

The researchers used the Hubble Space Telescope to precisely measure the galaxies’ sizes. The Very Large Telescope and Keck Telescope were then used to determine the speeds of the stars orbiting within the galaxies. With the sizes and star speeds in hand, the astronomers were then able to determine the galaxies’ masses and confirm their classification as UCDs.

UCDs may help scientists understand how galaxies in densely populated regions are transformed and destroyed. The team hopes to make additional observations to determine whether UCDs are dense nuclei of once-larger galaxies that have been stripped to the core by close interactions with giant galaxies in the cluster.