Researchers think that two of the closest galaxies to the Milky Way, the Large and Small Magellanic Clouds (LMC and SMC respectively), were caught in a bit of a fender bender a few hundred million years ago. Astronomers were studying the motion of stars in the SMC and discovered that part of the galaxy is moving away from the rest, a strong sign of a past collision.
The team used the European Space Agency’s Gaia telescope to study the internal dynamic of this satellite of the Milky Way. Gaia is designed to image stars again and again over several years. It is capable of such precise observations that it can work out how much stars have moved, even in the SMC which is almost 200,000 light-years from Earth.
The study, soon to be published in The Astrophysical Journal Letters, focused on massive stars that might have been ejected from the clusters where they formed. But the team discovered that stars from the southeast region of the small galaxy, known as the “Wing”, are all moving away from the main body. Although models have previously suggested there was an interaction, this is the first unambiguous finding that shows that the two Magellanic Clouds collided in recent cosmic time.
“This is really one of our exciting results. You can actually see that the Wing is its own separate region that’s moving away from the rest of the SMC,” lead author Professor Sally Oey, from the University of Michigan, said in a statement. “We’ve been looking at very massive, hot young stars – the hottest, most luminous stars, which are fairly rare. The beauty of the Small Magellanic Cloud and the Large Magellanic Cloud is that they’re their own galaxies, so we’re looking at all of the massive stars in a single galaxy.”
In 2012, co-author Dr Gurtina Besla and colleagues created a model of possible interactions that could explain the structures in the two small galaxies. Their model predicts that if the Magellanic Clouds had directly collided in their past, the Wing should be moving toward the LMC. This is exactly what has been observed. The stars in the Wing are escaping the SMC at roughly 64 kilometers (40 miles) per second.
There are 59 small galaxies in the neighborhood of the Milky Way and dozens of them are confirmed satellites of our galaxy. The Magellanic Clouds are the only ones visible to the naked eye, and while they are named after the 16th-century Portuguese explorer Ferdinand Magellan, they've been known about since prehistory in Africa, Australasia, and South America.