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Were the 2 Magellanic Clouds once 3?

A new study suggests that the Large and Small Magellanic Clouds – small satellite galaxies to our Milky Way – might have once had a 3rd companion.

View larger. | A meteor streaks between the 2 Magellanic Clouds. Photo by EarthSky friend Colin Legg of Australia.

New research suggests that two of the closest galaxies to the Milky Way — the Large Magellanic Cloud and the Small Magellanic Cloud — might have once had a third companion. The study, published September 18, 2018 in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society, describes how this third galaxy was likely engulfed by the Large Magellanic Cloud some three to five billion years ago.

Benjamin Armstrong is a masters student at the International Centre for Radio Astronomy Research (ICRAR) and the lead author on the study. He said most stars in the Large Magellanic Cloud revolve clockwise around the center of the galaxy. But, unusually, some stars revolve counter-clockwise. Armstrong said in a statement:

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.

The team used computer modeling to simulate galaxy mergers. Armstrong said:

What we found is that in this sort of merging event, you actually can get quite strong counter-rotation after a merger takes place. This is consistent with what we see when we actually observe the galaxies.

The Large Magellanic Cloud photographed using a small telephoto lens and a modified camera to highlight the molecular clouds, or regions of star formation. Image via Andrew Lockwood.

From the Southern Hemisphere, the Magellanic Clouds can be seen in the night sky with the unaided eye. There are mentions of them in the stories of ancient cultures, from thousands of years ago. The Large Magellanic Cloud is a relatively small 160,000 light-years away from us, while the Small Magellanic Cloud is around 200,000 light-years away.

Armstrong said his team’s new computer simulation could also help explain a problem that’s perplexed astronomers for years: why stars in the Large Magellanic Cloud are generally either very old or very young. He said:

In galaxies, there are these large objects called star clusters. 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.

Astronomers call this the ‘age-gap’ problem, Armstrong said, adding:

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 research was about asking pertinent questions that astronomers could start examining.

It’s about creating a new idea, a new way of looking at an old problem.

The Large and Small Magellanic Clouds, visible from Earth’s Southern Hemisphere. Image via Andrew Lockwood.

Bottom line: A new study suggests that the Large Magellanic Cloud and the Small Magellanic Cloud might have once had a third companion.

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Eleanor Imster

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