New Discovery Unravels Small Magellanic Cloud's Unusual Stellar Motion

The Small Magellanic Cloud (SMC), a galactic neighbor of the Milky Way, has been an object of extensive astronomical study for over half a century. A long-standing puzzle has been the non-standard orbital motion of its stars, which do not orbit the galaxy's center in the typical manner observed in most galaxies.

Collision with Large Magellanic Cloud Identified as Cause

A study published in The Astrophysical Journal by University of Arizona astronomers indicates that a direct collision between the SMC and the Large Magellanic Cloud (LMC) is responsible for the SMC's unusual stellar motion.

Himansh Rathore, a graduate student at Steward Observatory and the lead author, described the observation as witnessing "a galaxy transforming in live action."

Impact on Stellar and Gas Dynamics

Previous measurements using the Hubble Space Telescope and the Gaia satellite had confirmed the SMC's stars lacked typical orbital rotation. The research suggests that a collision several hundred million years ago, where the SMC passed through the LMC's disk, disrupted the SMC's internal structure. The LMC's gravity caused disordered motion among the SMC's stars, and the LMC's gas exerted pressure that eliminated the SMC's gas rotation.

For decades, observations suggested the SMC's gas was rotating. However, this study proposes that this was an illusion caused by the viewing angle; the collision stretched the SMC, making gas moving towards and away from Earth appear to be rotating.

Implications for Astronomical Research

This finding challenges the long-held status of the SMC as a benchmark for understanding galaxy formation and evolution across cosmic time.

Gurtina Besla, a professor at Steward Observatory and senior author, stated that the SMC is "not a 'normal' galaxy by any means" due to the catastrophic crash.

Methodology and Related Findings

The research team utilized computer simulations tailored to the known properties of the SMC and LMC, including gas content, star mass, and relative positions to the Milky Way. They also applied theoretical calculations to model the effects of the SMC's passage through the LMC's dense gas. New methods were developed to interpret scrambled stellar motions in post-collision galaxies.

Another study by the team, published in 2025, indicates that the collision also affected the LMC, tilting its central bar-shaped structure. The degree of this tilt may offer a new method to measure the amount of dark matter within the SMC, a substance inferred through gravitational effects but not yet directly detected.