The Small Magellanic Cloud (SMC), one of the closest galaxies to the Milky Way, is a compact, gas-rich galaxy visible to the naked eye from the Southern Hemisphere. It orbits our galaxy along with its larger companion, the Large Magellanic Cloud (LMC), and all three have been interacting for hundreds of millions of years. Because of its proximity, the SMC has been studied in detail for decades, with astronomers mapping its star, tracking its gas, and measuring its motion.
Still, one fundamental mystery remained unsolved. Unlike most galaxies, the stars in the SMC do not move in regular orbits around the center, and scientists are searching for an explanation.
The collision explains the star’s loss of rotation.
New research published in astrophysical journal shows a dramatic answer. A team from the University of Arizona discovered that the SMC’s unusual behavior is the result of a direct collision with the Large Magellanic Cloud. The discovery also raises concerns about using SMCs as a standard example for understanding how galaxies formed and evolved over cosmic time.
“We are witnessing a live-action transformation of galaxies,” said Himanshu Rathore, a graduate student at Steward Observatory and lead author of the paper. “SMC gives us a unique perspective with a front row seat to something deeply transformative, a process that is important to how galaxies evolve.”
Gases, gravity, and broken motion
The SMC contains more mass of gas than star. Under normal conditions, gas cools under gravity and settles into a rotating disk, much like the process that forms the flat rotating surface of our solar system. However, previous measurements using the Hubble Space Telescope and the European Space Agency’s Gaia satellite showed that the stars in the SMC do not follow this expected pattern.
Rasool said the likely cause was a collision hundreds of millions of years ago. During this event, the SMC passed directly through the LMC’s disk. The gravity involved destroyed the SMC’s structure and scattered its stars in chaotic motion. At the same time, the dense gas in the LMC exerted strong pressure on the gas in the SMC, effectively depriving it of its rotation.
“Imagine sprinkling water droplets on your hand and moving them through the air. As the air passes through, the water droplets are blown away by the pressure. Something similar happened when the gas in the SMC penetrated the LMC,” Rasool said.
Solving decades-old illusions
This study also resolves a long-standing contradiction regarding SMC gases. Observations over the years have suggested that the gas inside galaxies is rotating. Because stars are formed from gas and typically inherit that motion, astronomers expected stars to rotate as well. But that wasn’t what they observed.
New analysis shows that this apparent rotation was misleading. Collisions stretch the SMC, and gas moving along this stretched shape toward or away from Earth can appear rotated when viewed from certain angles.
Rethinking space benchmarks
For decades, the SMC has served as an important reference point for studying how galaxies form and evolve stars. These new findings cast doubt on that role.
“The SMC experienced a catastrophic collision that injected a huge amount of energy into the system. It is by no means a ‘normal’ galaxy,” Besra said.
To reach these conclusions, the researchers used detailed computer simulations that matched known properties of both galaxies, including their gas content, stellar mass, and position relative to the Milky Way. They combined these models with theoretical calculations to understand how gas in the SMC behaves as it moves through the dense environment of the LMC. The research team also developed a new technique to interpret the scrambling motion of stars in galaxies that have experienced collisions.
This is important because the SMC’s small size, high gas content, and low abundance of heavy elements made it an important comparison for galaxies in the early universe. If it is still recovering from a large collision, it may no longer serve as a reliable model.
Clues about dark matter caused by galaxy collisions
The collision could also yield new insights into dark matter. In another study published in 2025, the same research team found that the collision left a visible trace in the Large Magellanic Cloud. Its central bar-like structure is tilted from the galactic plane, a feature associated with the collision.
Rasool explained that the extent of this tilt depends on the amount of dark matter contained in the SMC. This relationship provides a new way to estimate dark matter, which cannot be directly observed and is detected only through gravitational effects.
“We’re used to thinking of astronomy as a snapshot in time,” Rasool says. “But these two galaxies got so close that they passed through each other and turned into something else.”
