MCELS / Cerro Tololo Inter-American Observatory / University of Michigan
The Small Magellanic Cloud
The Small Magellanic Cloud (SMC) is one of the Milky Way’s closest galactic neighbors. It is a small gas-rich galaxy, visible to the naked eye from the Southern Hemisphere, and which remains gravitationally linked to our galaxy.
Alongside SMC is its companion, Large Magellanic Cloud (LMC), and these three galaxies have been influencing each other for hundreds of millions of years.
According to , stars in the SMC do not orbit their center in the usual way seen in most galaxies, and scientists have had difficulty explaining this behavior.
In a new one, published last month in The Astrophysical Journalthe researchers found that the absence of stellar rotation in the SMC likely results from a direct collision with the LMC.
The discovery also raises concerns about using the SMC as a model to understand the evolution of galaxies over cosmic time.
“We are watching a galaxy transform in real time. SMC offers us a view of something very transformative in a process that is fundamental to how galaxies evolve,” says the study’s first author, .
The SMC contains more mass in gas than in stars. Under normal conditions, the gas cools, contracts due to gravity, and forms a rotating disk, similar to the process that created the flat, rotating structure of our solar system.
However, previous measurements using the Hubble Space Telescope and the satellite Gaia from the European Space Agency showed that the SMC stars do not move in an orderly rotation around the center of the galaxy.
The team of scientists points to a past collision as the likely cause. Millions of years ago, the SMC appears to have passed directly through the LMC disk. The gravitational forces involved disturbed the structure of the SMC, sending its stars into a chaotic and disorganized movement. At the same time, the LMC gas exerted intense pressure on the SMC gas, depriving it of its rotation.
“This situation also helps explain a long-standing enigma about SMC gas,” says the study’s co-author, .
For years, observations suggested that the galaxy’s gas was rotating. Since stars form from gas and inherit its motion, scientists expected stars to rotate as well.
The researchers performed computer simulations designed to match the known properties of both galaxies, including their gas content, total stellar mass e positions relative to the Milky Way. They combined these models with calculations that describe how the SMC’s gas behaved as it moved through the LMC’s dense environment during the collision.
They then developed techniques for interpreting the disordered motions of stars in a galaxy that has experienced such an event, and these methods can help better understand what telescopes are actually observing in the SMC.
In another, published in 2025, experts found that the collision also left a visible mark on the LMC. The galaxy has a central bar-shaped structure that is tilted away from its main plane, a feature associated with past interaction.
Furthermore, they observed that the stars in the SMC were moving in opposite directions on both sides of the galaxy, as if they were being pushed away.
“These two galaxies came very close, crossed each other and transformed into something different”, concludes Rathore.
Thus, the present investigation shows that the apparent rotation was misleading. The collision stretched the SMC, and gas moving toward Earth and away along this structure can create the illusion of rotation when viewed from certain angles.
