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There is a huge halo of gas circling our Galaxy with the mass of a billion Suns.

Stripped from the two dwarf Magellanic galaxies that orbit the Milky Way, the enormous stream of gas spreads over more than a quarter of the southern night sky. Yet it is only now that astronomers can explain why the Magellanic Stream persists and is as massive as it is.

The researchers publish their findings today in the journal .

Over billions of years, the Milky Way has captured smaller galaxies in its orbit and the two largest are known as the Small and Large Magellanic Clouds. Easily visible to the naked eye in the Southern Hemisphere, they are home to billions of stars and circle each other in an elaborate dance as they speed around the Milky Way.

For 50 years astronomers were puzzled as to why both dwarf galaxies could leave behind a huge trail of gas with as much mass as a billion Suns. This Magellanic Stream is moving through the Milky Way鈥檚 hot gas halo and it should have faded away, like the condensation trail behind an aircraft.

Now, an international team of astronomers has found evidence that both Magellanic Clouds are themselves surrounded by huge hot haloes as well that protect the gas stream from the Milky Way. Their new model accounts for the huge mass in the gas stream.

Co-author of the study聽Professor Joss Bland-Hawthorn, Director of the聽, said: 鈥淲e are fortunate here in the Southern Hemisphere that we can see these nearby galaxies with the naked eye. Now we know that these stellar smudges are surrounded by hot, huge, gassy spheres that overlap each other.

鈥淚f humans could see in the ultra-violet and X-ray spectra, we could see these remarkable structures emitting plasma across much of the night sky.鈥�

Outdated models

The scientists found that as the smaller galaxies were pulled into the Milky Way鈥檚 influence, parts of their halo were stretched and dispersed to form the Magellanic Stream.

鈥淭he existing models of the formation of the Magellanic Stream are outdated because they can鈥檛 account for its mass,鈥� said lead author聽, a PhD student at the University of Wisconsin-Madison in the USA.

His supervisor at Wisconsin-Madison,聽, said: 鈥淭hat鈥檚 why we came out with a new solution that is excellent at explaining the mass of the stream.鈥�

Older models suggested that gravitational tides and the force of the galaxies pushing against one another formed the Magellanic Stream as the dwarf galaxies came into orbit around the Milky Way. While these models could explain the stream鈥檚 size and shape, they accounted for just one-tenth of its mass.

Recently, astronomers discovered that the Magellanic Clouds are massive enough to have their own halo, or corona, of warm gas enveloping them. Professor D鈥橭nghia and her team realised this corona would dramatically alter how the stream formed.

鈥淭he stream is a 50-year puzzle,鈥� said co-author聽, an astronomer at the Space Telescope Science Institute in Baltimore, USA, which operates the Hubble Space Telescope. 鈥淲e never had a good explanation of where it came from. What鈥檚 really exciting is that we鈥檙e closing in on an explanation now.鈥�

In new simulations carried out by Mr Lucchini, the creation of the Magellanic Stream is divided into two periods. While the Magellanic Clouds were still far away from the Milky Way, the Large Magellanic Cloud stripped gas away from its smaller partner over billions of years. This stolen gas ultimately contributed 10 to 20 percent of the final mass of the stream.

Later, as the clouds fell into the Milky Way鈥檚 orbit, the corona gave up one-fifth of its mass to form the Magellanic Stream, which was stretched across an enormous arc of the sky by interactions with the Milky Way鈥檚 gravity and its own corona.

Reservoir for future stars

The new model is the first to explain the full mass of the Magellanic Stream and the vast majority that comes from ionized gas, which is more energetic than non-ionized gas. It also better explains how the stream adopted its unusual shape and why it lacks stars 鈥� because it was formed largely from the star-free corona, not the dwarf galaxies themselves.

鈥淭his work refines our understanding of how gas accretes onto the Milky Way and forms the reservoir for future star formation,鈥� said Professor Bland-Hawthorn in the聽School of Physics聽at the University of Sydney.

The researchers鈥� model can now be directly tested. The Hubble Space Telescope should be able to see the tell-tale signatures of the corona of gas surrounding the Magellanic Clouds. If confirmed, the findings will help explain a half-century mystery about the origin of the stream, offering a fuller picture of our galactic neighbourhood.

Declaration

Joss Bland-Hawthorn is an Australian Research Council Laureaute Fellow. Elena D鈥橭nghia acknowledges the hospitality of the聽聽at the Flatiron Institute in New York, USA, during the completion of this work.