AUSTRALIA: Astronomers discover galactic 'freak'
The discovery, made by a team of five Australian and three North American astronomers, was published in the March Monthly Notices of the Royal Astronomical Society.
It is the closest ultra-compact dwarf galaxy to Earth. It is far brighter and more massive than the clusters of stars that usually surround galaxies, and was born in the very early stages of the formation of the universe.
Astronomers from Australia's Swinburne University, Dr George Hau and Professor Duncan Forbes, said the galaxy was discovered when using a California Institute of Technology telescope in Hawaii.
"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," Hau said.
"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."
Hau said there was a lot of debate about how such galaxies formed. "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."
As well as being unusually small, the ultra-compact dwarf galaxy, named SUCD1, is very old - perhaps 10 billion years, indicating it was formed in the early stages of the universe. Also, it appears to consist mainly of stars, rather than dark matter, which dominates the mass of most galaxies.
The astronomers also found that SUCD1 emits a powerful stream of X-rays. 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," said Forbes.