The Large and Small Magellanic Clouds
Originally uploaded by Ken Schwarz

The are the nearest neighbours to our own galaxy – the Large Magellanic Cloud is about 160,000 light years away, and contains about 10 billion stars (about 1/10 the number of stars in the Milky Way). Its smaller companion, the Small Magellanic Cloud, is about 200,000 light years away and is about 100 times smaller than the Milky Way. They can't be seen from the UK, but are easily visible in the night sky south of the equator. They are named after the explorer Ferdinand Magellan, who observed them while circumnavigating the globe between 1519 and 1522.

The conventional view of the Magellanic Clouds has been that they are small satellite galaxies orbitting the Milky Way. There are some observations which seem to support this. Firstly, the gaseous disc of the Milky Way is warped, extending up to 10,000 light years above and below the plane of the galaxy. The warping could have been caused by the gravitational influence of the Magellanic Clouds, on previous passages past our galaxy. Also, a stream of hydrogen gas, known as the Magellanic Stream, extends through the Magellanic Clouds across about 100 degrees of the night sky. Models of the stream suggest that it has been formed by tidal interactions between the Magellanic Clouds and the Milky Way, and shaped by ram pressure between the gaseous halo of the Milky Way and interstellar gas in the clouds.
However, a team from the United States have now measured the velocities of the Magellanic Clouds and found them to be anomalously high. The conclusions are that either a) the Milky Way is much more massive than was previously thought or b) the Magellanic Clouds are not gravitationally bound to the Milky Way after all. Further analysis and computer modelling favours the second conclusion – the parabolic orbit of the Magellanic Clouds indicates that they are on their first pass past the Milky Way, having arrived between 1 and 3 billion years ago.
Given these new observations, we now have a mystery. How did the Milky Way become warped, and where did the gas in the Magellanic Stream come from?
The full paper is Besla et al. “Are the Magellanic Clouds on their First Passage about the Milky Way?”.