A tiny and extremely rare galaxy sits at the edge of the much larger and well-known Sombrero Galaxy.
Credit: Hubble Space Telescope
SYDNEY: A weird and tiny galaxy has been discovered, which lacks dark matter and must have been born in the very early stages of the universe, but astronomers are debating how such an object could form.
It's so weird that its discoverers are calling it a "freak". Technically though, the galactic object is an 'ultra-compact dwarf galaxy' named SUCD1. These are far brighter and more massive than the clusters of stars that usually surround galaxies, and this discovery may furnish the missing link in understanding how galaxies and their clusters evolve.
The galaxy was discovered with the mighty 10-metre Keck II telescope on the mountain of Mauna Kea in Hawaii by four Australian astronomers, and their results were published in the March 2009 Monthly Notices of the Royal Astronomical Society.
Galactic evolution
"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," astronomer George Hau, from Swinburne University in Melbourne.
"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 said.
How such rare phenomena form is a mystery, but the discovery of SUCD1 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," Hau explains.
Unusual properties
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. 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 Duncan Forbes, from Swinburne University. "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," said Forbes. "It is an exciting time to be working in this field."
- with Swinburne University
