A stellar discovery

29 May 2003

UNSW scientists are part of an international research team that has discovered a new type of galaxy, known as an "ultra-compact dwarf galaxy" (UCD).

The galaxies are so compact that astronomers previously mistook them for nearby stars in galaxy censuses based on large, ground-based pictures taken of the sky.

The finding was made by a team of eight astrophysicists from Australia, the U.S., Germany, and the U.K. The UNSW team members were Professor Warrick Couch and Dr Kenji Bekki, both from the school of physics. Project team leader was Dr Michael Drinkwater, previously with UNSW and now senior lecturer in physics at the University of Queensland.

"The results reported in Nature are the culmination of work done on the world's most powerful telescopes to establish that a distinct and new class of galaxy, not previously seen, has been discovered," Professor Couch says.

"Importantly, our observations have confirmed that UCDs are brighter and more massive than the biggest conglomerations of stars that are seen around galaxies.

"Hence they appear to be galaxies in their own right, but are very unusual in terms of their faint and very compact nature," he says.

The UCDs were first discovered with the 3.9m Anglo-Australian Telescope (AAT) at Siding Spring Observatory, Coonabarabran. The researchers were observing all objects in the direction of the Fornax Cluster - a bound system of about 300 galaxies that is 60 million light years away.

Amongst the 2500 objects that had previously been identified as stars, seven were actually found to be members of the Fornax cluster. At this distance they were too bright to be stars, but rather had to be a new class of extremely compact 'dwarf' galaxy, which had never been seen before.

The researchers then won rare time on the Hubble Space Telescope to measure how big these dwarf galaxies were, as well as time on the European Southern Observatory's Very Large Telescope in Chile to measure how fast stars are moving around inside these galaxies.

These two measurements are used in combination by astronomers to "weigh" galaxies and find out how massive they are. This confirmed the UCDs to be a new type of low-mass galaxy held together by gravity.

"This raises the question as to how such galaxies could have been formed, and work done at UNSW has been instrumental in providing that understanding," Professor Couch says.

Dr Kenji Bekki, an ARC Research Associate at UNSW, has used a GRAPE supercomputer to simulate how galaxies respond to the very strong gravitational forces they experience when orbiting the giant bright galaxies seen in the Fornax Cluster.

"They show that a UCD is most likely the 'naked nucleus' of a galaxy that has had its outer regions stripped by these forces," Dr Bekki says.

"The discovery of this new type of galaxy is a significant step forward in confirming that gravity can severely modify the structure and appearance of galaxies and identifying what the 'end products' of this process are."

Team members were: Dr Michael Drinkwater of UQ: Dr Michael Gregg of the University of California at Davis, and the Lawrence Livermore National Laboratory in California; Dr Michael Hilker of Bonn University; Dr Kenji Bekki and Professor Warrick Couch of UNSW; Dr Harry Ferguson of the Space Telescope Science Institute in Baltimore; Dr Bryn Jones of the University of Nottingham; and Dr Steven Phillipps of the University of Bristol.

The work has received Australian Research Council funding of $180,000 for a two-year program.


Front page image: The Fornax Cluster of galaxies with insets, 60-times magnified, of a normal dwarf galaxy (upper) and a newly-discovered "ultra-compact dwarf" (UCD) galaxy (lower). A simulation of the remains of a normal dwarf galaxy after disruption to form a UCD is shown in the centre.