There鈥檚 something strange in the neighbourhood, and it may be a sign of a nearby galaxy that we didn鈥檛 know existed.
The mystery starts with two dwarf galaxies called the Magellanic Clouds that orbit the Milky Way, and a stream of gas in front of them that is tough to explain. Past attempts at puzzling out the origin of this object, known as the Leading Arm, point to gravitational interactions between the Magellanic Clouds. But Marcel Pawlowski at the Leibniz Institute for Astrophysics Potsdam in Germany and his colleagues now say that is聽probably wrong.
Pawlowski鈥檚 team simulated the behaviour of the gas and included the diffuse corona of hot gas that surrounds the Milky Way, which was not part of most previous models. With the corona included, they were unable to replicate the Leading Arm, because interactions between the corona鈥檚 gas and that of the Magellanic Clouds stopped it from forming.
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鈥淭he fact that we see this Leading Arm and our current best understanding of the Milky Way doesn鈥檛 match with that is really puzzling,鈥� says Pawlowski.
One potential explanation is that we simply do not understand the Milky Way鈥檚 corona very well. The Leading Arm could survive if the outer part of the corona is about 10 times less dense than we think it ought to be, but that would throw a wrench into our ideas about our galaxy鈥檚 mass, some of which we think comes from that coronal gas.
The other explanation is that, rather than coming from the Magellanic Clouds, the Leading Arm actually comes from another galaxy just ahead of them that has been stripped of gas as it plunges into the Milky Way.
That idea has a problem too: the researchers checked all the galaxies in the area that we know of, and none of them match this 鈥渇orerunner鈥� galaxy 鈥� they tended to be too far ahead of the Magellanic Clouds. If our galaxy does have a secret neighbour, we鈥檒l have to keep on searching for it.
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