Over 50 small galaxies have been discovered surrounding the Milky Way. The two most famous ones are the Large and Small Magellanic Clouds, which are visible to the naked eye in the Southern Hemisphere. The two of them are very slowly interacting and falling into the Milky Way, leaving behind an enormous trail of gas: the Magellanic stream.

The stream is composed of high-velocity clouds and has a combined mass of over 1 billion times the mass of the Sun, which has long puzzled astronomers. The two satellite galaxies are clearly responsible for its formation but how and why the stream is as massive as it is was not understood. The stream is moving through the Milky Way’s hot gas halo and should have faded away instead of persisting.

Researchers have now produced a radical proposal. A new paper in Nature suggests the existence of a halo (corona) of hot gas surrounding the Large Magellanic Cloud itself. This Magellanic Corona interacts with the halo that surrounds the Milky Way, creating the stream.

Their simulations suggest that the stream formed in two distinct episodes. When the clouds were further away from the Milky Way, the Large Magellanic Cloud stripped its smaller companion of some of its gas. This theft would account for up to 20 percent of the stream's mass. Another 20 percent would be material from the Magellanic Corona itself when at a later time, the two galaxies entered the halo of the Milky Way. The remaining mass comes from the halo of our own galaxy. 

“The existing models of the formation of the Magellanic Stream are outdated because they can’t account for its mass,” lead author Scott Lucchini, a graduate researcher at the University of Wisconsin-Madison, said in a statement.

“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,” added Elena D’Onghia, who supervises Lucchini.

The team has estimated that the Magellanic Corona has a temperature of about 500,000°C (900,000°F). The scientists believe it should be able to be observed and studied similarly to how Andromeda’s halo was, using distant quasars to act as a backlight, providing a way to illuminate the diffused corona. Observations are crucial to confirm if this explanation is indeed the correct one. The Hubble Space telescope should be capable of performing such observations.

“The stream is a 50-year puzzle,” added Andrew Fox, one of the co-authors of the study and an astronomer at the Space Telescope Science Institute, which operates 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.”

You may remember the stream being in the press recently when researchers discovered in the recent past (cosmically speaking) it had probably been visible in the sky to our ancient ancestors after becoming extremely bright due to a flare from the Milky Way's supermassive black hole.