Two of the nearest galaxies to the Milky Way, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), are easily spotted by eye in the southern hemisphere. Sometimes shortened as “The Clouds,” these much smaller and irregular-shaped “dwarf” galaxies orbit about the Milky Way, and will eventually be assimilated by it.
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We have known for a while now of a column of hydrogen gas and stars that connects these two dwarf galaxies. But therein also lies the mystery: where did this stream of material come from? The two close contenders are: close fly-bys of the Clouds with the Milky Way, or close fly-bys of the LMC with the SMC.
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Clues to solve this mystery come from studying the stars near the outer edges of The Clouds, which are held more tentatively as a result of the weaker gravitational pull.
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Astronomer Dougal Mackey and collaborators (Australian National University) have undertaken a census of stars in the outskirts of the LMC and SMC as a part of a larger project called the “The Dark Energy Survey.” The result of their census is changing the way we think about The Clouds.
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Their census revealed that the stars separate out into two populations: an “intermediate age” 1 billion year one, and an “ancient” approximately 11 billion year one. In other words, they found different generations of stars to occupy different locations within the SMC, as if the stars followed zoning laws requiring older stars to live in the stellar equivalent of “retirement” communities.
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This result hints that this “Magellanic Stream” may not be a result of a close approach to the Milky Way, but rather may have been a feature of the The Clouds for billions of years.
