Weighing the Large Magellanic Cloud Using Stellar Streams

How massive is the Large Magellanic Cloud, one of our closest galactic neighbours? And where exactly did that odd name come from? To answer the first question, astronomers have used data from the Anglo-Australian Telescope and looked at the influence the galaxy has on the Milky Way itself. To answer the second, I did the research so you don’t need to. 

We’ll get to the name later, but let’s talk about the science first. The Large Magellanic Cloud (LMC) is an irregularly-shaped conglomeration of stars about 163,000 light-years from Earth. That might sound like a long way, but for a galaxy, it is relatively close. The Andromeda galaxy, for example, is 2.5-million light-years from us.

The LMC and its smaller (and more distant) partner, the Small Magellanic Cloud, are generally classified as satellite galaxies of the Milky Way. Astronomers used to think that they both orbited our galaxy, but as it turns out they are probably moving too fast for that to be true. If they are orbiting the Milky Way, a single orbit must take at least 4 billion years.

That’s not all we thought we knew that we now know we’d didn’t really know. Assumed for a long time to be galaxies without any discernible structure, once astronomers got hold of telescopes powerful enough to take a close look, they found that both galaxies have a bar across their centres.

That means they may once have been barred spiral galaxies, smaller versions of the Milky Way, and have since been disrupted by gravitational interactions with their more massive cousin. And like the Milky Way, they have unsettled other smaller galaxies and globular clusters as they’ve passed by.

It is by studying these gravitational interactions with dwarf galaxies and star clusters in the Milky Way halo that astronomers have managed to get the data they need to measure the mass of the LMC. But why would they want to do such a thing?

Understanding the masses of galaxies helps us to understand the formation and growth of the Universe. Dark matter remains a bit of a mystery, but scientists can learn more by comparing the total mass of galaxies to the amount that we can see. Precisely knowing the mass of a galaxy also helps us learn about its dynamic history and future.

What we do know about the dynamics of the LMC now is that it is influencing the behaviour of stellar streams around the Milky Way. But some predicted effects have not yet been observed. For example, one study found that the LMC could induce a substantial sideways velocity on a stream called Tucana III.

Stellar streams are like a cosmic necklace of stars, the filamentous remains of small galaxies and star clusters that have been strung out by the gravitational pull of the Milky Way. And there are lots of streams, many of them discovered in just the last five or six years.

The Southern Stellar Stream Spectroscopic Survey (S5) is a survey to map the motion and chemistry of stellar streams in the Southern Hemisphere using the Anglo-Australian Telescope at Siding Spring Observatory. The survey began in 2018. 

Using data from S5 as well as Gaia (an astronomical mission creating a precise three-dimensional map of the Milky Way) and the Dark Energy Survey (an international effort to map hundreds of millions of galaxies), an international collaboration of astronomers, several of whom are at universities in Australia, had a close look at the influence of the LMC on stellar streams around the Milky Way.

Previous research had shown that the motion of the streams was different to their tracks. That’s a sure sign that they have been gravitationally disturbed.

What the team found is that they could accurately model the streams if the LMC had a mass of about 180 billion suns. For context, the Milky Way galaxy has a mass of about 1,500 billion suns. Keep in mind that these masses include the enigmatic dark matter that is part of both galaxies.

Their results are consistent with other measurements of the LMC mass with stellar streams, but an order of magnitude larger than when the mass is based on the dynamics of stars within the galaxy alone. It’s possible that by looking at the dynamics of stars alone a big chunk of mass in the halo is being missed.

The good news is that the Vera C. Rubin Observatory, which will be used to survey even more distant streams, is due to see first light in 2022. Once the results come in from its Legacy Survey of Space and Time (LSST) there will be additional data for astronomers to use to refine their models.

Now, I did say at the beginning of this article that I’d explain where the name for the Magellanic Clouds came from. 

It had something to do with the man who led what turned out to be the first circumnavigation of the Earth from a Spanish port on 20 September 1519. Magellan never made it back to his home in Portugal, having been killed in the Philippines, but he later became renowned for his navigational skill and tenacity in leading what was one of the most important maritime voyages ever undertaken.

But what of the Clouds? Early Arabic explorers navigated using the stars as they explored to the south in search of new trade routes. But the Clouds were too diffuse to be used in navigation.

The first Europeans to see the southern sky were probably Portuguese sailors in the 15th century, but without an application to navigation on the open seas, interest in the Clouds would have been limited. At that time, astronomers and navigators were more interested in finding a southern star analog of Polaris to accurately determine latitude in the Southern Hemisphere.

Of course, no such star exists. When Magellan sailed in the early 16th century, mapping the night sky was not a priority, and no reports from the expedition even mention the Clouds.

Even 100 years later, the Clouds were being referred to by their Latin names of Nubecula Major and Nubecula Minor. For some reason, over the years, sailors began referring to them as the Magellanic Clouds, and the name didn’t reach public and scientific prominence until perhaps the late 19th century.

Of course, it would be remiss not to mention that, although we have adopted the European name for the Clouds, they have a rich tradition in the cultures of Aboriginal Australians going back tens of thousands of years.

So, Magellan neither discovered nor even used the Clouds for navigation during his historic journey, but he nevertheless has his name attached to some of the most unique objects we can see in the night sky without a telescope.