Astronomers may have finally explained why the Small Magellanic Cloud isn’t spinning like it should. The cause: it’s still reeling from an ancient run-in with its big sister, the Large Magellanic Cloud.
In a new study published on Monday in the Astrophysical Journal, researchers used computers to simulate the two dwarf galaxies’ 100-million-year-long collision. They charted the Large Magellanic Cloud’s impact on its smaller sibling to answer the long-standing question of why the latter’s stars aren’t orbiting as fast as they should.
“Understanding of the inner 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 in Italy, who wasn’t involved in the new paper. Any study to help explain this “is a step in the direction of solving the puzzle of complex interactions between galaxies.”
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Galaxies contain hundreds of millions of stars and are suffused in sparse, ambient clouds of gas and dust as well. For a galaxy with a given size and brightness, the rotation of both the stars and the disk of gas follow a reliable pattern.
A simulated collision between the Small Magellanic Cloud and the Large Magellanic Cloud that was used in a new study to explain why stars in the former are not rotating as fast as they should be.
Himansh Rathore/University of Arizona
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—20 degrees from its neighbor, the Large Magellanic Cloud. (The two galaxies are named after Ferdinand Magellan, even though it was actually one of his crew who reported them, and he wasn’t the first European to see the objects.)
In the past few decades, astronomers have noticed that the stars in the smaller galaxy aren’t swirling around its center as fast as they should be, especially when compared with the rotation of its disk of gas. To explain this, they’ve floated the idea that we’re looking at the Small Magellanic Cloud after it ripped through the Large Magellanic Cloud, disrupting the former’s rotation.
To test this, a team of four scientists built a computer simulation of this hypothetical collision. It produced snapshots of the two galaxies before, during and after the crash, a time frame that spanned hundreds of millions of years. The violence, they found, left the smaller galaxy in disarray, with its stars and gas rotating far less than before.
The Small Magellanic Cloud (SMC) is often used as a comparison point for galaxies in the early universe because it’s poor in heavy metals. So this dramatic update to its cosmic history might lead astronomers to revisit those comparisons.
“The SMC went through a catastrophic crash that injected a lot of energy into the system,” says Gurtina Besla, an astronomer at Steward Observatory and a co-author of the study. “It is not a ‘normal’ galaxy by any means.”
