CAUGHT RED-HANDED: OUR GALAXY
IS DESTROYING ITS NEIGHBOURS

The finding is published today in the journal Nature (paper). It will also be announced this afternoon at an international conference at the Australian Academy of Science in Canberra, hosted by the Australian National University's Mount Stromlo Observatory.

The two nearest neighbouring galaxies, the Large and Small Magellanic Clouds (left), are less than a tenth of our Galaxy in size. They are close by, only twice as far from the starry edge of our Galaxy as that edge is from the Galaxy's centre. And they orbit our Galaxy, in thrall to its gravitational pull.

That pull is now tearing them apart, by tugging harder on their near side than on their far side. Hydrogen gas stripped from the Clouds by this process is now streaming out ahead and behind them.

The dismemberment was imaged by the new 'multibeam' instrument (right) on the Parkes telescope, which detects the hydrogen gas in galaxies - the 'bones' underlying their starry 'flesh'.

Developed by CSIRO, it is the most powerful instrument of its kind in the world. Most of its work is searching for faint and hidden galaxies that can't be detected any other way.

The new picture has settled a long-standing controversy sparked by a much earlier finding made with the Parkes telescope.

In 1973 Dr (later Professor) Don Matthewson from Mount Stromlo Observatory used the Parkes telescope to discover a trail of gas flowing behind the Magellanic Clouds. This trail, called the Magellanic Stream, is more than twice as long as the diameter of our Galaxy.

Astronomers have argued for decades about the Stream's origin. "But over the years we've whittled it down to two choices," said CSIRO's Dr Lister Staveley-Smith, lead scientist on the multibeam project.

"One was a process called ram-pressure stripping - gas being swept out of the Magellanic Clouds as the Clouds travelled through our Galaxy's outskirts."

"The other was so-called tidal forces from our Galaxy. That idea predicted we'd find material coming off the leading edge of the Clouds - and we have," said Dr Staveley-Smith.

"People have looked before for this stuff but haven't found it because they sampled the sky rather coarsely, whereas we've been searching with a fine-toothed comb," said Ms Mary Putman, the PhD student at Mount Stromlo Observatory who made the key picture.

"We are interested in how galaxies interact, because it's an important aspect of how they evolve over time. We are still trying to understand how important interactions are in creating new stars, for instance," said Mt Stromlo Observatory's Dr Brad Gibson, Ms Putman's thesis supervisor.

* Distances to the Magellanic Clouds: the Large Magellanic Cloud is 160 000 light-years away (about 1.5 million million million kilometres). The Small Magellanic Cloud is 190 000 light-years away (about 1.9 million million million kilometres).

For more information:

• Dr Lister Staveley-Smith, CSIRO Australia Telescope National Facility.
• Dr Brad Gibson, Mount Stromlo and Siding Spring Observatories, Australian National University.
• Dr Rachel Webster, University of Melbourne, Project leader, multibeam HIPASS survey.
• Professor R.D. Ekers, Director, CSIRO Australia Telescope National Facility.