The Large and Small Magellanic Clouds — the largest and most well-known dwarf galaxies in the Milky Way’s orbit — are connected by a 43,000 light-year-long bridge of stars, according to a study recently published in the Monthly Notices of the Royal Astronomical Society.

The Large and Small Magellanic Clouds (LMC and SMC), dwarf irregular galaxies visible with the naked-eye in the southern hemisphere, reside 160,000 light-years and 210,000 light-years from us, respectively.

Even though these two galaxies have been a constant fixture of the heavens, astronomers have only recently had the chance to study them in any detail.

Whether the Magellanic Clouds fit the conventional theory of galaxy formation or not depends critically on their mass and the time of their first approach to the Milky Way?

An international team of astronomers found clues that could help answer both of these questions.

The team, led by Cambridge’s Institute of Astronomy researchers, concentrated on the area around the Magellanic Clouds and used data from ESA’s Gaia mission to pick out variable stars of a particular type — RR Lyrae stars.

As these stars have been around since the earliest days of the Magellanic Clouds’ existence, they offer an insight into the pair’s history.

Firstly, the RR Lyrae stars detected by the Gaia spacecraft were used to trace the extent of the LMC.

The LMC was found to possess a fuzzy low-luminosity ‘halo’ stretching as far as 20 degrees from its center. The galaxy would only be able to hold on to the stars at such large distances if it was substantially bigger than previously thought, totaling perhaps as much as a tenth of the mass of the entire Milky Way.

An accurate timing of the Clouds’ arrival to the Milky Way is impossible without knowledge of their orbits.

Unfortunately, satellite orbits are difficult to measure: at large distances, the object’s motion in the sky is so minute that it is simply unobservable over a human lifespan.

In the absence of an orbit, the astronomers found the next best thing: a stellar stream.

Streams of stars form when a satellite — a dwarf galaxy or a star cluster — starts to feel the tidal force of the body around which it orbits.

The tides stretch the satellite in two directions: towards and away from the host. As a result, on the periphery of the satellite, two openings form: small regions where the gravitational pull of the satellite is balanced by the pull of the host.

Satellite stars that enter these regions find it easy to leave the satellite altogether and start orbiting the host. Slowly, star after star abandons the satellite, leaving a luminous trace on the sky, and thus revealing the satellite’s orbit.

“Stellar streams around the Magellanic Clouds were predicted but never observed,” said Dr. Vasily Belokurov, the lead co-author on the study.

“Having marked the locations of the Gaia RR Lyrae on the sky, we were surprised to see a narrow bridge-like structure connecting the Clouds.”

“We believe that at least in part this ‘bridge’ is composed of stars stripped from the SMC by the LMC.”

“The rest may actually be the LMC stars pulled from it by the Milky Way.”

The astronomers believe the RR Lyrae bridge will help to clarify the history of the interaction between the Magellanic Clouds and our Galaxy.

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Vasily Belokurov et al. 2017. Clouds, Streams and Bridges: redrawing the blueprint of the Magellanic System with Gaia DR1. Mon Not R Astron Soc 466 (4): 4711-4730; doi: 10.1093/mnras/stw3357

This article is based on a press-release from the University of Cambridge.