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Cosmic tails betray a close encounter


The Small (left) and Large (right) Magellanic Cloud galaxies orbit together around our Milky Way galaxy. A large stream of gas, not visible in this image, stretches between the two galaxies, the result of a close encounter around a billion years ago.

OUR NEAREST GALACTIC NEIGHBOURS became entangled in a cosmic dance over the past few billion years, with a dramatic close encounter around 1.2 billion years ago, say astronomers.

International Centre for Radio Astronomy Research (ICRAR) astronomers Jonathan Diaz and Dr Kenji Bekki have used computer modelling to study the movement of the Large and Small Magellanic Clouds around the Milky Way and the structure of the gas that surrounds them.

The Large Magellanic Cloud and the Small Magellanic Cloud are the two closest reasonable size galaxies to our own Milky Way. Southern Hemisphere stargazers can easily see them in the night sky from dark locations.

“An enormous stream of hydrogen gas trails behind the Magellanic Clouds as they orbit the Milky Way,” says ICRAR student Jonathan Diaz. ICRAR is a joint venture between Curtin University and The University of Western Australia, located in Perth.

Animation of the Magellanic Stream

Simulation of the orbits of the Large and Small Magellanic Cloud galaxies (red and green lines) around the Milky Way. A close approach around a billion years ago was responsible for forming a huge cloud of gas around the galaxies.

“Previous explanations for the oversized tail had it being stripped away from the Magellanic Clouds during a close approach of the Milky Way around 2 billion years ago.”

However, recent observations made by the Hubble Space Telescope have cast doubt on whether that close approach actually occurred. The new data from Hubble shows that the Magellanic Clouds are moving differently than originally thought.

“We have found a solution to the question raised by the Hubble data,” explains Diaz. “We’ve shown that its possible for the gas stream to form through a violent interaction between the two small galaxies around 1.2 billion years ago, without the need for a strong interaction with the much larger Milky Way.”

“Past models have assumed that the Magellanic Clouds have been cosmic companions since birth, but our work demonstrates a recent and quite dramatic coupling between the Clouds.”

“Our model shows the Magellanic Clouds have been drifting around the Milky Way for many billions of years, but have only just recently found each other,” says Dr Kenji Bekki, supervisor of the project.

“Were going to conduct further simulations and refine our model but this result shows us we still have more to learn about our galaxy and its neighbourhood.”

The research will be published in the Monthly Notices of the Royal Astronomical Society.

Adapted from information issued by ICRAR. Images courtesy Jonathan Diaz (ICRAR) / Eckhard-Slawik / Serge Brunier.

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