The Small Magellanic Cloud’s stars are all mixed up NASA/ESA/A. Nota (STScI/ESA)

The Milky Way’s closest neighbours may just be a couple of passers-by having an argument.

The Large and Small Magellanic Clouds are dwarf galaxies about 200,000 and 160,000 light years away. Trailing behind them is a huge band of hydrogen gas called the Magellanic Stream. Since its discovery in 1974, astronomers have viewed it as evidence that the two galaxies are orbiting the Milky Way’s poles, leaving the gas in their wake.

But more recent observations show the galaxies are moving fast enough to escape the galaxy’s grip, suggesting that they are not locked into an orbit. What’s more, they may have arrived on our doorstep only in the past 3 billion years.


“This new scenario is reasonable because the Magellanic Clouds are very different from all other dwarf galaxies that surround the Milky Way,” says Bruno Dias at the European Southern Observatory in Santiago, Chile.

That raised another question: if the galaxies aren’t orbiting the Milky Way, then what is stripping their stars away to form the Magellanic Stream?

Polling the neighbours

To find out whether the clouds are residents or visitors, Dias and his colleagues studied star clusters within the clouds to figure out their ages, star types and locations. Like sample voters in political polls, star clusters are good proxies for the galaxies’ overall contents. “Star clusters act like fossils full of information from the past evolution of the clouds,” Dias says.

Dias and his team found the oldest stars in the Small Magellanic Cloud are further away from the galaxy’s centre than expected. This suggests another galaxy has been exerting its influence. They also found young stars that should be in more central regions of the galaxy but had moved further out.

The work supports the idea that the Large Magellanic Cloud is stripping stars and gas away from the Small Magellanic Cloud, without the Milky Way’s involvement, says Nitya Kallivayalil at the University of Virginia in Charlottesville. That supports the idea that they’re new in town.

“The Large Magellanic Cloud and the Small Magellanic Cloud have been a pair for a long time, and have been perturbing each other even when they were far away from the Milky Way,” she says. “Figuring out their past trajectories with respect to each other, not just with the Milky Way, is where there is a lot of interest in the community.”

The results raise several new questions that need answering before we can tell whether the clouds are permanent residents or merely on holiday, Dias says, such as the origin of individual star clusters within the clouds and how fast they are rotating.

The work also offers insights on how galaxies form and evolve, he adds. Astronomers often look at a large number of distant galaxies to learn about their evolution, but studying a smaller number of close-up galaxies can help work out how these structures interact with each other.

“A few galaxies can be studied in much greater detail,” he says.

Journal reference: arxiv.org/abs/1604.03086