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There’s something odd about our galactic neighbourhood, which Sidney van den Bergh at the Herzberg Institute of Astrophysics in Canada highlights today in a short paper.

Astronomers have long known that the Milky Way’s two closest neighbours are the Large and Small Magellanic Clouds, giant clouds of stars, gas and dust called irregular galaxies.

This is strange for two reasons. These galaxies are much younger than ours and may have even formed together. It looks as if they may just be passing by, on their way to somewhere else. Most other galaxies like ours, such as Andromeda, don’t have a single companion like this, so having two seems rather fortunate.

But there’s something else as well. The Large Magellanic cloud is unusually luminous. In fact, there are only two other irregular galaxies in the entire local universe that come close. “In other words the Large Magellanic Cloud seems to be close to the upper luminosity limit for irregular galaxies,” says van den Bergh. That’s unusual too.

In recent years, astronomers have begun to work out just how rare this is. Sky surveys such as the Sloan Digital Sky Survey allow astronomers to work out the distribution of various types of galaxy. They’ve looked at 22581 galaxies like the Milky Way and found that 81% have no satellite galaxies as bright as the Magellanic Clouds, 11% have one such satellite, and only 3.5% host two such satellite galaxies.

That makes the Milky Way very unusual. As van den Bergh puts it: “That the Galaxy should have an irregular companion as luminous as the Large Magellanic Cloud is almost a miracle.”

One of the central tenets of cosmology is the Copernican principle: that we live on an ordinary planet, in an average galaxy, in a mediocre part of the Universe.

But it’s beginning to look as if the Milky Way, or at least its neighbourhood, doesn’t follow that rule at all. So the question for astronomers and cosmologists is why this has come about and what its significance should be. An interesting conundrum.

Ref: arxiv.org/abs/1012.3492: A Strange Mènage Á Trois

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