Astronomers have suspected for some time that the Large and Small Magellanic Clouds collided in the recent past. The Gaia space telescope provides striking new evidence for a head-on collision.

Aside from their closeness and brightness, the Magellanic Clouds don’t appear to be anything special — they’re a pair of dwarf irregular galaxies (read: blobs) orbiting the Milky Way and linked by a “Magellanic Bridge” of gas and stars, pulled loose from the duo by gravitational forces. But the past decade has seen astronomers’ understanding of the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) completely overthrown, resulting in new revelations about the violent and ongoing formation of our Milky Way.

Now, a team led by Sally Oey (University of Michigan) has, serendipitously, made a discovery that bears out predictions that these two galaxies once collided: A big chunk of the SMC is moving away from the main body of the galaxy, toward the Magellanic Bridge and the LMC.

Astronomical wisdom once held that the LMC and SMC had been orbiting the Milky Way for billions of years, but in 2007, Gurtina Besla (then at the Harvard-Smithsonian Center for Astrophysics) led a landmark study demonstrating that the dwarf galaxies are likely falling into the Milky Way’s gravity for the first time, on their way to being consumed by our much larger galaxy. That realization introduced a new puzzle — if the Magellanic Bridge wasn’t stripped away from the LMC and SMC by the powerful gravity of the Milky Way, how did it get there?

In 2012, Besla’s team offered an answer. The Magellanic Bridge was formed by gravitational forces between the LMC and SMC themselves. The two galaxies must have had a close encounter in the recent past — perhaps even a direct collision — to pull the Bridge’s stars and gas away so strongly.

In the November 1st Astrophysical Journal Letters, Oey and her collaborators presented support for the direct collision hypothesis based on an analysis of new data from the Gaia satellite, which is measuring the three-dimensional positions and velocities of a billion stars in the Milky Way and its satellites.

Oey’s team wasn’t initially looking for evidence of a collision at all. Rather, they were looking for young, massive, fast-moving “runaway” stars in the SMC to learn how these stars achieve such high velocities. But in the process, they discovered that many of the stars in the eastward “wing” of the SMC were moving in concert, a result that can only be explained by the LMC’s gravitational force acting on the galaxy globally.

Astronomers who study the Magellanic pair are excited by the results. The next step will be to see how narrowly the bulk motion of the SMC wing constrains the geometry of the recent LMC–SMC collision, says Sarah Pearson (Flatiron Institute), who studies interacting pairs of dwarf galaxies.

“I think that’s one of the most interesting things coming out now with Gaia, that hopefully people will start to compare this exact observational evidence to the models,” says Pearson. “If the LMC and SMC did have a direct encounter, if the SMC is plowing through the LMC disk, you would see this bulk motion.” If instead they had a close flyby but not a direct encounter, Pearson explains, you’d expect a different sort of bulk motion, perpendicular to the Magellanic Bridge instead of toward it.

With further study, astronomers might be able to use the observed bulk motion to understand the direct collision in even more detail: how long ago it occurred, at what speed, and at what angle. The better astronomers understand the past trajectories of these galaxies, the more precisely they can predict their future paths, which will see them accumulated into the Milky Way’s stellar halo.

References:

M. S. Oey et al. "Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud." Astrophysical Journal Letters. November 1, 2018.

G. Besla et al. "Are the Magellanic Clouds on Their First Passage about the Milky Way?" Astrophysical Journal. October 20, 2007.

G. Besla et al. "The Role of Dwarf Galaxy Interactions in Shaping the Magellanic System and Implications for Magellanic Irregulars." Monthly Notices of the Royal Astronomical Society. April 11, 2012.