A nearby galaxy long treated as a stable cosmic reference is revealing a far more dynamic and turbulent history.
The Small Magellanic Cloud, or SMC, is one of the Milky Way’s nearest galactic neighbors. It is a small, gas-rich galaxy visible to the naked eye from the Southern Hemisphere, and it remains gravitationally linked to our galaxy along with its companion, the Large Magellanic Cloud, or LMC.
These three galaxies have influenced each other for hundreds of millions of years. The SMC, in particular, has been studied in great detail.
Astronomers have spent decades cataloging its stars, mapping its gas, and tracking how it moves. Despite this extensive research, one key mystery has persisted. The stars in the SMC do not orbit its center in the usual way seen in most galaxies, and scientists have struggled to explain this unusual behavior.
A Galaxy in Motion—and Disruption
A study published in The Astrophysical Journal offers a possible answer. Researchers from the University of Arizona found that the SMC’s lack of stellar rotation likely stems from a direct collision with the LMC. This discovery also raises concerns about using the SMC as a model for understanding galaxy evolution over cosmic time.
“We are seeing a galaxy transforming in live action,” said Himansh Rathore, a graduate student at Steward Observatory and the study’s lead author. “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. Under normal conditions, gas cools, contracts due to gravity, and forms a rotating disk, similar to the process that created the flat, spinning structure of our solar system. However, earlier measurements using the Hubble Space Telescope and the European Space Agency’s Gaia satellite showed that the SMC’s stars are not moving in an orderly rotation around the galaxy’s center.
Evidence of a Cosmic Collision
Rathore and his colleagues point to a past collision as the likely cause. Several hundred million years ago, the SMC appears to have passed directly through the disk of the LMC. The gravitational forces involved disrupted the SMC’s structure, sending its stars into chaotic, disorganized motion. At the same time, the LMC’s gas exerted intense pressure on the SMC’s gas, stripping away its 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.
This event also helps explain a long-standing puzzle about the SMC’s gas, according to Gurtina Besla, an astronomy professor at Steward Observatory and the study’s senior author. For years, observations suggested that the galaxy’s gas was rotating. Since stars form from gas and inherit its motion, scientists expected the stars to rotate as well.
The new research shows that the apparent rotation was misleading. The collision stretched the SMC, and gas moving toward and away from Earth along this stretched structure can create the illusion of rotation when viewed from certain angles.
Rethinking a Cosmic Benchmark
For decades, astronomers have used the SMC as a reference point for studying how galaxies form stars and evolve. This new work suggests that assumption may need to be reconsidered.
“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.
To investigate, the team ran detailed computer simulations designed to match the known properties of both galaxies, including their gas content, total stellar mass, and positions relative to the Milky Way. They combined these models with calculations describing how the SMC’s gas behaved as it moved through the LMC’s dense environment during the collision.
The researchers also developed new techniques to interpret the scrambled motions of stars in a galaxy that has experienced such an event. These methods can help scientists better understand what telescopes are actually observing in the SMC.
Implications for Dark Matter and Galaxy Evolution
The findings have wider implications. Because the SMC is small, rich in gas, and low in heavy elements, it has often been used as a stand-in for galaxies in the early universe. However, a system still affected by a major collision may not serve as a reliable comparison.
A separate study by the same team, published in 2025, found that the collision also left a visible imprint on the LMC. The galaxy has a central bar-shaped structure that is tilted out of its main plane, a feature linked to the past interaction. Rathore, who led that study, explained that the tilt depends on how much dark matter the SMC contains. This provides a new way to estimate dark matter, which has never been directly observed and is known only through its gravitational influence.
“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.”
Reference: “A Galactic Transformation—Understanding the SMC’s Structural and Kinematic Disequilibrium” by Himansh Rathore, Gurtina Besla, Roeland P. van der Marel and Nitya Kallivayalil, 16 March 2026, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ae4507
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