Commonwealth Scientific and Industrial Research Organisation (CSIRO)

AUGUST 20, 1998


Astronomers using CSIRO's Parkes radio telescope have made a picture that shows our Galaxy ripping apart its nearest neighbours.

The finding is published today in the journal Nature. 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, 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 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 Mathewson 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).



Interaction of our Galaxy with the Large and Small Magellanic Clouds

Image 1:

Artist's impression: our Galaxy's gravity claws at the fabric of two neighbouring galaxies, the Large and Small Magellanic Clouds, pulling out streams of hydrogen gas.

The letters X, Y and Z label the axes, to indicate positions in 3D space.

Picture: Dallas Parr, CSIRO. Media are welcome to use the graphic if they are following up CSIRO media release 98/194 and wish to illustrate the story.

Image 2:

Sweeping around the bottom of this picture is part of our own Galaxy. Above it lie the Large and Small Magellanic Clouds (the blue blobs, left and right respectively). Between those galaxies and ours runs a newly-discovered archipelago of hydrogen gas -- the small red and green blobs -- torn from the Magellanic Clouds by our Galaxy's gravity.

This image, covering 2 400 square degrees of sky, is a world first. It was made with the multibeam system on CSIRO's Parkes radio telescope by Mary E. Putman and colleagues. The colour-coding shows variations in the intensity of the radio waves from the various regions. The blue areas put out the strongest radio waves, the red areas the weakest.

Image 3: The same picture without the color-coding and with the various features labelled.

Download a quicktime simulation showing the orbits of the Large and Small Magellanic Clouds around our Galaxy. The present day "now" is marked by a pause in the sequence. 6.29KB or 33.4KB

Image 4:

Screenshot from the quicktime video simulation showing the position of the Large and Small Magellanic Clouds now.

Image 5:

Screenshot from the quicktime video simulation showing the position of the Large and Small Magellanic Clouds as they are likely to be in the future.

The quicktime simulation was produced by P.S. Li, University of Wyoming and Harvey A. Thronson, NASA, with support from NASA and NCSA. Copyright 1998 Board of Trustees, University of Wyoming.

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