Massive Halo Might Explain What’s With the Stream of Gas Around the Milky Way

The Milky Way is not the only object around in its neighborhood; it has pulled in its orbit smaller galaxies, and the two largest are called the Small and Large Magellanic Clouds. These two cosmic bodies are visible as twin dusty stains in the Southern Hemisphere.

​As the Magellanic Clouds started to orbit the Milky Way some billions of years ago, a massive stream of gas known as the Magellanic Stream was torn from them. The stream now expands across over half of the night sky.

Still, astronomers have no explanation yet regarding the reason why the stream became as large as it is now – more than a billion times the mass of the Sun.

Protection Against Gravity

Now, experts at the University of Wisconsin-Madison and their colleagues have found that a halo of warm gas around the Magellanic Clouds​ most probably acts like a protective cocoon, protecting the dwarf galaxies from the Milky Way’s own halo and providing most of the Magellanic Stream’s mass. As the smaller galaxies were pulled into the sphere of the Milky Way’s influence, regions of this halo were expanded and dispersed to form the Magellanic Stream.

“The existing models of the formation of the Magellanic Stream are outdated because they can’t account for its mass,” says Scott Lucchini, a graduate student in the UW-Madison physics department, first author of the paper.

“That’s why we came out with a new solution that is excellent at explaining the mass of the stream, which is the most urgent question to solve,” adds Elena D’Onghia, a professor of astronomy at UW-Madison who supervised the research.

Older models implied that gravitational tides and the force of the galaxies pressing against one another shaped the Magellanic Stream​ out of the Magellanic Clouds as the dwarf galaxies came into orbit around the Milky Way. Although these models could explain the stream’s size and shape in general, they covered only a tenth of its mass.

The Magellanic Stream’s Formation

Not long ago, astronomers discovered that the Magellanic Clouds are sufficiently large to have their own halo, or corona, of warm gas surrounding them. D’Onghia and her team figured that this corona would massively change the way the stream formed.

In new models carried out by Lucchini, the formation of the Magellanic Stream​ is split into two periods. While the Magellanic Clouds were still at a great distance from the Milky Way, the Large Magellanic Cloud pulled in gas from its smaller partner throughout billions of years. The stolen gas eventually accounted for 10 to 20 percent of the final mass of the stream.

A view of the gas in the Magellanic System as it would appear in the night sky. The Magellanic Corona covers the entire sky while the Magellanic Stream is seen as gas flowing away from the two dwarf galaxies. [Image Credit: Colin Legg/Scott Lucchini]
Later, as the clouds dropped into the Milky Way’s orbit, the halo gave up a fifth of its own mass to shape the Magellanic Stream​, which was expanded across a massive arc of the sky by engagements with the Milky Way’s gravity and its own corona.

The new simulation is the first to justify the full mass of the Magellanic Stream​ and the colossal majority that comes from ionized gas, which is more dynamic than non-ionized gas. It also better explains the way the stream ended up with its filamentous shape and why it doesn’t host stars – because it was formed mainly from the star-free halo, not the dwarf galaxies themselves.

A ’50-Year Puzzle’

“The stream is a 50-year puzzle,” says Andrew Fox, one of the co-authors of the research and an astronomer at the Space Telescope Science Institute, which manages the Hubble Space Telescope. “We never had a good explanation of where it came from. What’s really exciting is that we’re closing in on an explanation now.”

The scientists’ proposal can now be directly tested, and the Hubble Space Telescope should be able to observe the revealing signatures of the halo of gas surrounding the Magellanic Clouds​.

“This work redefines our understanding of how gas accretes onto the Milky Way and forms the reservoir for future star formation,” says Joss Bland-Hawthorn, a co-author of the paper and director of the Sydney Institute for Astronomy in Australia.

The scientists published their discoveries on September 9th in the journal Nature.​

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