Something extremely strange is happening to our galactic neighbor. Scientists think they know why

Something extremely strange is happening to our galactic neighbor. Scientists think they know why

By Joseph Howlett edited by Claire Cameron

Astronomers may have finally explained why the Small Magellanic Cloud isn’t spinning like it should. The reason: it’s still raging after an ancient collision with its big sister, the Large Magellanic Cloud.

In a new study published Monday i Astrophysical Journalscientists used computers to simulate the two dwarf galaxies’ hundred-million-year-long collision. They mapped its impact on its smaller sibling to answer the long-standing question of why stars don’t orbit as fast as they should.

“Understanding the internal structure and dynamical state of the Small Magellanic Cloud is a long-standing struggle in the field,” says Michele De Leo, an astronomer at the University of Bologna. Any study to help explain this, he says, “is a step toward solving the puzzle of complex interactions between galaxies.”

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Galaxies contain hundreds of millions of stars, and are also permeated by sparse, surrounding clouds of gas and dust. For a galaxy of a given size and luminosity, the rotation of both the stars and the gas disk follows a reliable pattern.

The Small Magellanic Cloud is a conspicuous exception. It’s a small, elongated galaxy that people in the Southern Hemisphere can see regularly in the night sky—twenty degrees from its neighbor, the Large Magellanic Cloud. (They are named after Ferdinand Magellan, although it was actually one of his crew who reported them, and he was not the first.)

Over the past few decades, astronomers have noticed that the stars in the smaller galaxy are not swirling around the center as quickly as they should be, especially compared to the rotation of the gas disk. To explain this, they have come up with the idea that we are looking at the Small Magellanic Cloud after it tore through the large one and disrupted its rotation.

To test this, a team of four researchers built a computer simulation of this hypothetical collision. It produced snapshots of the two galaxies before, during and after the crash, spanning hundreds of millions of years. The violence, they found, left the smaller galaxy in disarray, with stars and gas rotating far less than before.

The Small Magellanic Cloud is often used as a point of comparison for galaxies in the early universe, as it is poor in heavy metals. So this dramatic update to its cosmic history may prompt astronomers to return to those comparisons.

“The SMC went through a catastrophic crash that injected a lot of energy into the system,” says Gurtina Besla, an astronomer at the Steward Observatory and the study’s senior author. “It is by no means a ‘normal’ galaxy.”

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