The Small Magellanic Cloud or the SMC is one of the Milky Way's closest galactic neighbors – a small, gas-rich galaxy visible to the naked eye from the southern hemisphere, bound to our galaxy by gravity, alongside its companion, the Large Magellanic Cloud or the LMC. All three galaxies have been interacting for hundreds of millions of years. The SMC is also one of the most studied galaxies in the sky. Astronomers have catalogued its stars, mapped its gas and tracked its motion for more than half a century. Yet a basic question about it has remained. The galaxy's stars do not orbit around its center the way stars in most galaxies do, and it has been challenging to explain why.
In a study published in The Astrophysical Journal, University of Arizona astronomers traced the lack of rotation in stars to a direct collision between the SMC and its larger companion, the Large Magellanic Cloud, or the LMC. The findings also raise questions about how scientists use the SMC as a reference point for understanding galaxies across the history of the universe.
"We are seeing a galaxy transforming in live action," said Himansh Rathore, a graduate student at Steward Observatory and the lead author of the paper. "The SMC gives us a unique, front-row view of something very transformative of a process that is critical to how galaxies evolve."
The SMC contains more mass in gas than in stars. Gas cools, contracts under gravity and settles into a rotating disk, the same process that shaped the spinning plane of our solar system. But when researchers, including those at U of A, previously measured the motion of the SMC's stars using the Hubble Space Telescope and the Gaia satellite of the European Space Agency, the SMC's stars were not orbiting around the galaxy's center the way stars in most galaxies do.
The possible reason, Rathore said, is a collision. A few hundred million years ago, the SMC crashed directly through the LMC's disk. The LMC's gravity disrupted the SMC's internal structure and sent its stars into random, disordered motion. Also, the LMC's gas applied a tremendous amount of pressure to the SMC's gas and destroyed its gas rotation.
"Imagine sprinkling water droplets on your hand and moving it through the air – as the air rushes past, the droplets get blown off because of the pressure it exerts. Something similar happened to the SMC's gas as it punched through the LMC," Rathore said.
The collision accounts for a decades-old puzzle about the SMC's gas, said Gurtina Besla, an astronomy professor at Steward Observatory and the study's senior author. For decades, telescope observations suggested that the gas inside the SMC was rotating. But stars form out of gas and inherit its motion, which means, if the gas were spinning, the stars would typically be too. But the researchers now show the rotation was an illusion of viewing angle – the collision is stretching the SMC, and gas moving toward and away from the Earth along that stretch looks like rotation from certain perspectives.
For decades, the SMC served as a benchmark for understanding how galaxies form stars and evolve across cosmic time, a status this study now puts in question.
"The SMC went through a catastrophic crash that injected a lot of energy into the system. It is not a 'normal' galaxy by any means," Besla said.
The team used computer simulations tailored to match the known properties of the SMC and the LMC – their gas content, total star mass, and positions relative to the Milky Way. They paired the simulations with theoretical calculations of how the SMC's gas was affected as it plowed through the LMC's dense gas environment during the collision. They also developed new methods for reading the scrambled star motions in a post-collision galaxy, tools that can now be used to properly interpret what telescopes actually measure in the SMC.
That matters because the SMC is small, gas-rich and low in heavy elements, which are properties that made it a standard yardstick for the kinds of galaxies that existed early in the universe. A galaxy still reeling from a collision may not be a clean reference point, Besla said.
Another study published by the team in 2025 showed that the collision also left a physical mark on the LMC that could help scientists probe dark matter. The LMC has a bar-shaped structure at its center, and that bar is tilted out of the plane of the galaxy because of the collision. Rathore, the 2025 study's lead author, said the degree of the tilt is tied to how much dark matter the SMC contains, giving researchers a new way to measure a substance that has never been directly detected, only inferred from its gravitational effects.
"We are used to thinking of astronomy as a snapshot in time," Rathore said. "But these two galaxies have come very close together, gone right through one another, and transformed into something different."
