How the Magellanic Stream was formed

The Milky Way does not travel through the universe alone. It is accompanied on its journey by smaller galaxies. The two largest are the Small Magellanic Cloud and the Large Magellanic Cloud, which are both visible as dusty twin smudges in the southern hemisphere.

When the Magellanic Clouds started orbiting the Milky Way billions of years ago (astronomers are not certain about the timing and the gravitational bond, it’s possible that they are still on their first approach to the Milky Way), an enormous stream of gas known as the Magellanic Stream was ripped out of them. The stream now extends across more than half the night sky. Astronomers, however, cannot explain how the stream became so massive that it contains a billion solar masses, which corresponds to one-tenth of the mass of the LMC.

Astronomers from the University of Wisconsin-Madison and their colleagues have now discovered that a halo made up of warm gas surrounding the Magellanic Clouds is probably acting something like a protective cocoon that shields the dwarf galaxies from the Milky Way’s own halo and accounts for the majority of the mass of the Magellanic Stream. When the smaller galaxies came into an area influenced by the Milky Way, parts of this halo were stretched out and scattered and formed the Magellanic Stream. The researchers published their results on September 9, 2020 in the journal, Nature.

“The existing models for the formation of the Magellanic Stream are obsolete, because they cannot explain its mass,” says Scott Lucchini, a doctoral candidate in the physics department at UW-Madison, lead author of the study. “That’s why we developed a new solution that can explain the mass of the stream very well,” adds Elena D’Onghia, a professor of astronomy at UW-Madison.

Older models assumed that gravitational tides and the force of the opposing galaxies formed the Magellanic Stream from the Magellanic Clouds as the dwarf galaxies entered an orbit around the Milky Way. While these models could largely explain the size and shape of the stream, they only produced a stream with a tenth of the mass. Recently, astronomers discovered that the Magellanic Clouds are so massive that are surrounded by their own halo or corona made of warm gas. D’Onghia and her team realized that this corona would dramatically change the formation of the stream.

In new simulations performed by Lucchini, the formation 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 pulled off gas 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 moved into orbit around the Milky Way, the corona gave up a fifth of its own mass to form the Magellanic Stream, which was stretched across a huge section of the sky due to interactions with the force of gravity from the Milky Way and its own corona.

The new model is the first that explains the full mass of the Magellanic Stream and its largest component, which comes from ionized gas that is more energetic than non-ionized gas. It also better explains how the stream got its thread-like shape and why it doesn’t have stars – because for the most part is was formed from the star-free corona and not from the dwarf galaxies themselves. “The Stream is a 50-year-old puzzle,” says 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’ve never had a good explanation for where it came from.”

The researchers’ theory can now be tested directly. The Hubble Telescope should be able to see the telltale signatures of the gas corona surrounding the Magellanic Clouds.

A view of the Magellanic Stream as it would look in the night sky if it were visible to the naked eye. The Magellanic Corona covers the entire sky, while the Magellanic Stream is seen as gas flowing from the two dwarf galaxies, the Large Magellanic Cloud and the Small Magellanic Cloud. This image comes directly from the numerical simulations but was slightly modified for aesthetic reasons. (image: Colin Legg/ Scott Lucchini).
The video shows the dynamics of the Magellanic Clouds as they approached the Milky Way over the last 1.34 billion years. The panel at the top shows all the gas, including the ionized Magellanic Corona, while the panel at the bottom shows only the cold gas originating from the disks of the clouds. Observational data is shown in the panel at the bottom (Video: Scott Lucchini)
The approach of the Magellanic Clouds toward the Milky Way. The observational data is shown on the left side, with the dots representing ionized gas, while the simulation is shown on the right side. The colors represent the line-of-sight velocity of the gas and the brightness indicates the relative density. The video starts 550 million years ago and lasts up to today. (Video: Scott Lucchini)

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