The Large Magellanic Cloud, a dwarf galaxy located about 160,000 light-years away, has a counter-rotating stellar population in its disk. In a new paper in the Monthly Notices of the Royal Astronomical Society: Letters, ICRAR astronomers Benjamin Armstrong and Kenji Bekki propose a scenario in which the origin of this stellar population is the result of a merger with another dwarf galaxy more than 3 billion years ago.
B. Armstrong & K. Bekki suggest that the Large (left) and Small Magellanic Clouds were originally a triplet system containing a companion dwarf galaxy. Image credit: Andrew Lockwood.
“Most stars in the Large Magellanic Cloud rotate clockwise around the center of the galaxy. But, unusually, some stars rotate anti-clockwise,” Armstrong said.
“For a while, it was thought that these stars might have come from its companion galaxy, the Small Magellanic Cloud.”
“Our idea was that these stars might have come from a merger with another galaxy in the past.”
In the study, the researchers used computer modeling 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,” Armstrong said.
“This is consistent with what we see when we actually observe the galaxies.”
This 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,” Armstrong 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,” he added.
“Because in the Large Magellanic Cloud we see star formation starting again, that could be indicative of a galaxy merger taking place.
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,” Armstrong said.
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B. Armstrong & K. Bekki. 2018. Formation of a counter-rotating stellar population in the Large Magellanic Cloud: a Magellanic triplet system? MNRAS 480 (1): L141-L145; doi: 10.1093/mnrasl/sly143
