Nearby dwarf galaxy being stretched and disrupted by larger neighbour
Researchers including a Keele scientist have found that a dwarf galaxy near to our own Milky Way is expanding and stretching due to the gravitational pull from its larger neighbour.
The international research team, including Keele’s Dr Jacco van Loon, were studying stellar motions in the Small Magellanic Cloud (SMC), a dwarf galaxy which orbits the Milky Way and is around 200,000 light years away.
Using data from the VISTA Survey of the Magellanic Clouds (VMC), researchers measured the motions of millions of stars across the Small Magellanic Cloud with unprecedented precision.
The new study, published in Astronomy & Astrophysics and led by the Leibniz Institute for Astrophysics Potsdam (AIP), provides direct evidence of a galaxy-wide tidal disruption of the Small Magellanic Cloud from its interaction with the Large Magellanic Cloud. Rather than showing coherent rotation typical of stable galaxies, stars across the Small Magellanic Cloud show large-scale outward motion, indicating the system is dynamically disturbed even in the inner regions.
The Small Magellanic Cloud is one of the Milky Way’s closest galactic neighbours and, together with the Large Magellanic Cloud, it forms a pair of interacting satellite galaxies visible from the Southern Hemisphere.
Because of their proximity, the Magellanic Clouds provide astronomers with a unique opportunity to study how galaxies evolve under the influence of gravity. Over time, interactions between the two galaxies have distorted their shapes, triggered bursts of star formation, and pulled streams of gas and stars into intergalactic space. The motions of stars preserve a record of these interactions. By tracking how stars move across the sky — known as “proper motions” — astronomers can reconstruct the dynamical history of the galaxy.
The findings reveal that stars across the Small Magellanic Cloud are moving outward along a southeast–northwest axis — a signature consistent with tidal stretching caused by the gravitational pull of the Large Magellanic Cloud. The team found that stars in the Small Magellanic Cloud are moving outward with an average speed of about 17 kilometres per second. At this rate, stars can be displaced by several thousand light-years over a few hundred million years, enough to significantly distort the galaxy’s structure.
Remarkably, the expansion is visible not only in the galaxy’s outskirts but also deep within its central regions. The researchers found no evidence for coherent rotational motion once tidal effects were properly accounted for. Instead, the observed stellar motions are predominantly radial, indicating that the Small Magellanic Cloud is in a strongly disturbed dynamical state.
The findings suggest that commonly used rotating-disk models oversimplify the true complexity of the galaxy’s internal dynamics. According to the study, such models can mistakenly interpret tidal streaming motions as rotation.
The study also uncovered a distinct northward stellar motion seen only among older red giant stars. This feature may preserve the imprint of an interaction that occurred more than two billion years ago. Younger and intermediate-age stars respond differently to the tidal forces, showing stronger and more coherent outward motions.
This population-dependent behaviour indicates that the Small Magellanic Cloud’s stellar populations retain memory of different stages of the galaxy’s interaction history.
Dr Jacco van Loon, Reader in Astrophysics at Keele and co-author of the study, said: "We are privileged to be witnessing a galaxy ripping its neighbour apart, on our doorstep. However, most of these stars will not be torn out of the SMC, which will be breathing a sigh of relief as it separates from the LMC."
