A University of Arizona team has determined that the Small Magellanic Cloud's chaotic star motions result from a collision with the Large Magellanic Cloud hundreds of millions of years ago. This impact disrupted the galaxy's structure and created an illusion of rotating gas. The findings, published in The Astrophysical Journal, challenge the SMC's role as a typical galactic example.
The Small Magellanic Cloud (SMC), a nearby companion to the Milky Way visible from the southern hemisphere, has long puzzled astronomers with its stars' disorganized orbits, unlike the orderly patterns in most galaxies. New research from the University of Arizona, led by graduate student Himansh Rathore at Steward Observatory, attributes this to a direct collision with the larger Large Magellanic Cloud (LMC) a few hundred million years ago. During the event, the SMC passed through the LMC's disk, scattering its stars and stripping rotation from its gas due to gravitational forces and pressure from the LMC's dense gas. Rathore compared it to water droplets blown off a hand moving through air: '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.' Earlier observations from the Hubble Space Telescope and ESA's Gaia satellite had shown no stellar rotation, contradicting apparent gas rotation, which the study explains as an optical illusion from the galaxy's stretched shape. Gurtina Besla noted, '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.' Researchers used computer simulations matching the galaxies' properties, including gas content and stellar mass, to model the collision's effects. This disrupts the SMC's use as a benchmark for early universe galaxies due to its high gas content and low heavy elements. A related 2025 study links the collision to the LMC's tilted central bar, offering clues to the SMC's dark matter content. Rathore remarked, 'We are seeing a galaxy transforming in live action.' The paper appears in The Astrophysical Journal (DOI: 10.3847/1538-4357/ae4507).