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March 2009 - Issue #5

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Astronomers discover galactic ‘missing link’

Story by Julian Cribb

View articles in related topics: Astrophysics/Astronomy

The discovery of a galactic freak – an extremely rare ultra-compact dwarf galaxy, the closest yet found to the Earth – could furnish the missing link in understanding how galaxies and their clusters evolve.

Swinburne University of Technology astronomers, led by Professor Duncan Forbes, discovered the dwarf galaxy, which is far brighter and more massive than the clusters of stars that usually surround galaxies. It also comes from a very early stage in the formation of the universe.

The object’s proximity has enabled scientists to make unusually detailed observations, finding it to be extremely old and confirming that it emits powerful X-rays pointing to violent processes raging within.

Using the mighty 10-metre Keck II telescope on Mauna Kea, Hawaii, the team of five Australian and three North American astronomers announced their unusual find in a letter to the Royal Astronomical Society.

“We were observing the properties of star clusters surrounding the well-known Sombrero Galaxy, when we detected this compact object that was far brighter than any of its companions,” explains lead author, Swinburne’s Dr George Hau.

They quickly calculated the object’s distance, finding it to be the same as the much larger Sombrero Galaxy – indicating that both lay about 33,000 light years (10 megaparsecs) from the Earth, and were clearly associated.

“It was only the size of a star cluster – which typically contain about one million stars – but it shone as brightly as a small galaxy. This indicated the object was an ultra-compact dwarf galaxy, a very unusual object, possibly containing 10 million stars,” he explains. (A regular galaxy has about 100 million stars. Our own galaxy, the Milky Way, has about 100 billion stars.)

How such rare phenomena form is a mystery, but the discovery of SUCD1, as the object has been named, has presented the perfect opportunity to find out, and to fill in another vital chapter in galactic evolution.

“There is much debate in the astronomical community about how these things form. The prevailing theory is that they are dwarf galaxies that have been stripped of their outer halo of stars by the gravitational forces of the large parent galaxy, leaving only the bright inner core of stars. But we think it may be something else: a massive star cluster that has formed independently,” Dr Hau says.

Ultra-compact dwarf galaxies were identified as a class only about 10 years ago, thanks to the growing power of modern optical telescopes. The new discovery was made only months after Swinburne’s Centre for Astrophysics and Supercomputing obtained exclusive viewing time on the giant 10-metre Keck II telescope, the most powerful optical instrument in the world, and is one of a string of important new findings that Swinburne’s scientists have since contributed to astrophysics.

Another unusual aspect of the ultra-compact dwarf galaxy is that it is very old – perhaps 10 billion years, indicating it was formed in the early stages of the universe, when things were all the more violent and energetic. Furthermore it appears to consist mainly of stars, rather than the still-enigmatic dark matter, which dominates the mass of most galaxies.

Small it may be, but SUCD1 is hardly peaceful, spitting out a powerful stream of X-rays, which the astronomers consider are probably being released while a black hole or neutron star strips and accretes matter from a companion star in a binary (two-star) system. The team believes this to be the first time that X-ray emissions have been clearly detected from an ultra-compact dwarf object.

“Based on all this evidence our interpretation is that SUCD1 is a massive star cluster that evolved on its own – rather than a stripped-down galaxy,” Professor Forbes says. “However, this is still very much a live debate at the moment.”

“These are exciting discoveries, and will certainly change the way we think about how ultra-compact dwarf galaxies and related objects form. Of course SUCD1 may be a special case, and obviously we need more examples in order to nail down its origin. At Swinburne we’re in a great position to find more such objects through our access to the Keck telescope. It is an exciting time to be working in this field.”

The Swinburne team involved in the discovery included Professor Forbes, Dr Hau, Lee Spitler, Dr Robert Proctor and Trevor Mendel.

Their publication ‘An Ultra Compact Dwarf around the Sombrero Galaxy (M104): the nearest massive UCD’ appeared in the Monthly Notices of the Royal Astronomical Society in early 2009.

View images of the Sombrero Galaxy.
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