If the Andromeda and Milky Way galaxies collide head-on when the two are predicted to merge five billion years from now, what is likely to happen to the Large and Small Magellanic Clouds?

The orbits of the Large and Small Magellanic Clouds will determine where they end up after the Milky Way and Andromeda galaxies merge. The Clouds’ orbits relate to the Milky Way’s mass, which has been a recent area of debate. A higher mass implies a more tightly bound orbit of the Large Magellanic Cloud (LMC; the Small Magellanic Cloud is coupled to its big brother). If the Milky Way is 1.75 trillion solar masses, then the LMC likely will remain bound to the resultant galaxy. If, instead, the Milky Way is only 1 trillion solar masses, the LMC has had a more unbound orbit and thus likely will be tossed into intergalactic space during the merger. Astronomy: Roen Kelly, after Genevieve Shattow and Abraham Loeb.

The Magellanic Clouds most likely will be tossed to an orbit that is farther away from the merger product of the Milky Way and Andromeda (so-called “Milkomeda”) than they are relative to the Milky Way today. At present, we know the Clouds are on a highly eccentric orbit relative to our galaxy. Although they are now near closest approach (the Large one is 160,000 light-years away while the Small Cloud is 200,000 light-years distant), astronomers expect them to spend most of their orbital time near the edge of our galaxy’s halo, almost 1 million light-years out. The kick that the Clouds will get from the merger between the Milky Way and Andromeda is extremely dependent on their exact location at the time of the merger.

As the two massive galaxies spiral toward each other, they might toss the Magellanic Clouds like a small piece of food thrown out of a blender. In this case, the Clouds would be lost to intergalactic space.

The Clouds may remain bound to Milkomeda, but even in that case they would move farther away from the center of the merger product. Making an exact prediction is difficult because we do not know the precise orbits of Andromeda and the Clouds, as they depend on the unknown mass distribution at the outskirts of the Milky Way and Andromeda. For example, the latest study (by Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues) found that the Milky Way has about twice as much mass as previously thought. A student of mine and I have written a paper that shows how this makes a significant change in the Clouds’ predicted orbit relative to the Milky Way (see above diagram). — Abraham Loeb, Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts

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