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Magellanic Clouds May Have Been a Trio

Two of the closest galaxies to the Milky Way – the Large and Small Magellanic Clouds – may have had a third companion.

Research published in the Monthly Notices of the Royal Astronomical Society (https://goo.gl/rrB6ZX) has described how another luminous galaxy was likely engulfed by the Large Magellanic Cloud 3–5 billion years ago.

Mr Benjamin Armstrong, a Masters student with the International Centre for Radio Astronomy Research who was lead author on the study, said most stars in the Large Magellanic Cloud rotate clockwise around the centre of the galaxy. Unusually, some stars rotate anti-clockwise.

“For a while, it was thought that these stars might have come from its companion galaxy, the Small Magellanic Cloud,” Armstrong said. “Our idea was that these stars might have come from a merger with another galaxy in the past.”

Armstrong used computer modelling to simulate galaxy mergers. “What we found is that in this sort of merging event you actually can get quite strong counter-rotation after a merger takes place,” he said. “This is consistent with what we see when we actually observe the galaxies.”

Armstrong said the finding could help to explain a problem that has perplexed astronomers for years – why stars in the Large Magellanic Cloud are generally either very old or very young. “In galaxies, there are these large objects called star clusters,” he said. “Star clusters contain many, many, many stars that are all of quite similar ages and made in similar environments.

“In the Milky Way, the star clusters are all very old. But in the Large Magellanic Cloud, we have very old clusters as well as ones that are very young – but nothing in between.” This is known as the age-gap problem.

“Because in the Large Magellanic Cloud we see star formation starting again, that could be indicative of a galaxy merger taking place,” Armstrong said.

The finding could also help explain why the Large Magellanic Cloud appears to have a thick disk. “Our work is still very preliminary but it does suggest that this sort of process could have been responsible for the thicker disk in the past,” he said.