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Two of the Milky Way's closest galaxies – big and small magellanic clouds – may have had a third companion, astronomers believe.
The research published today describes how another "bright" galaxy was probably swallowed up by the great Magellan cloud three or five billion years ago.
Benjamin Armstrong, principal student of the study, said that most of the stars of the great Magellan cloud were clockwise rotating around the center of the galaxy. But, exceptionally, some stars turn in the opposite direction of the needles of a clock.
"For a moment, we thought these stars could come from his companion galaxy, the Small Magellanic Cloud," Armstrong said. "Our idea was that these stars could come from a merger with another galaxy in the past."
Dr. Armstrong, based at the University of Western Australia, used computer modeling to simulate galaxy fusions.
"What we've found is that in this type of melting event, you can actually get a fairly strong counter-rotation after a merger," he said. "This is consistent with what we see when we actually observe galaxies."
Magellanic clouds can be seen at the naked eye in the night sky and have been observed by ancient cultures for thousands of years.
The large cloud of Magellan is relatively small, 160,000 light years away from us, while the small cloud of Magellan is about 200,000 light-years away.
Armstrong said the discovery could help explain a problem that has puzzled astronomers for years – why the stars of the Great Magellanic Cloud are usually very old or very young.
"In galaxies, there are these big objects called star clusters," he said. "Star clusters contain many, many, many stars that all have similar ages and are made in similar environments.
"In the Milky Way, the star clusters are all very old, but in the Great Magellanic Cloud we have very old groups as well as very young groups – but nothing in between."
That's what's called the "age difference" problem, Armstrong said.
"Because in the Great Magellanic Cloud we see star formation begin again, which could indicate a galaxy fusion," he said.
Mr. Armstrong said the discovery could also help explain why the big Magellan cloud seems to have a thick disc.
"Our work is still very preliminary but it suggests that this type of process could have been responsible for the thicker disk in the past," he said.
Dr. Armstrong said the research was intended to ask relevant questions that astronomers might begin to examine.
"It's about creating a new idea, a new way of seeing an old problem," he said.
The study was published in the journal Monthly Notices of the Royal Astronomical Society, published by Oxford University Press.
Reference: "Formation of a stellar population counter-rotating in the great Magellan cloud: a magellanic triplet system?" B. Armstrong & K. Bekki, 2018 September 18, Monthly Records of the Royal Astronomical Society [https://academic.oup.com/mnras, preprint (PDF): https://www.icrar.org/wp-content/uploads/2018/09/Magellanic-Triplet-System_MNRAS.pdf].
The International Center for Research in Radioastronomy (ICRAR) is a joint venture between Curtin University and the University of Western Australia, with support and funding from the Government of Western Australia.
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