Roughly 10 billion years ago the Milky Way—then a smaller galaxy that did not contain its current spiral structure or diffuse halo of surrounding stars—suffered a massive head-on collision that shook it to its very core.
That is when our galaxy’s gravity pulled a smaller companion, roughly one quarter its mass, into a dangerous dance: One where the dwarf galaxy plunged into and out of the Milky Way’s disk, oscillating back and forth until it was finally swallowed whole. Although our galaxy survived, it has never been the same. The collision scrambled the orbits of stars in its disk, making it much puffier, and sent alien stars flying all around the Milky Way, thus building much of its halo. The smash up also funneled new gas toward the galactic center, adding fuel that mixed and mingled with the Milky Way’s existing reservoirs to form new generations of stars.
Over time the dwarf galaxy faded away, but the scars from its collision never really disappeared—not that they have been easy to find. Astronomers, who have long thought the Milky Way likely grew from a vast number of merging dwarf galaxies, have struggled to uncover the signs of the largest mergers—until today. Now a new paper published in Nature provides proof—or something close to it. “It’s like uncovering a fossil or an archaeological piece of evidence for how the galaxy got started,” says James Bullock, an astronomer at the University of California, Irvine, who is unaffiliated with the new research.
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Co-authored by Amina Helmi, an astronomer at the Kapteyn Astronomical Institute in the Netherlands, and her colleagues, the paper is one of a torrent following the long-awaited second data release from Gaia—a spacecraft launched in 2013 by the European Space Agency to chart the heavens in unprecedented detail. Over the course of its mission Gaia has pinpointed the positions, motions, brightnesses and colors of 1.3 billion Milky Way stars with such accuracy that astronomers have used its data to craft new, exceptionally rich chapters in the biography of the Milky Way.
Indeed, after the second data set was released this April, many teams glimpsed hints of our violent history—recorded in stars that appeared out of place. Vasily Belokurov, an astronomer at the University of Cambridge, led a team that discovered a large number of stars moving out of sync with the galaxy’s rotation, instead streaming toward or away from the galactic center against all expectations. Another Kapteyn astronomer, Helmer Koppelman, helmed a study that spotted a “blob” of stars orbiting in the opposite direction to most of the stars in the Milky Way’s halo. And Misha Haywood, an astronomer at Paris Observatory, found fast-moving stars within the halo with atypical chemical abundances—a sign they may have formed outside the Milky Way. “It was really the tip of the iceberg,” Helmi says. Although all these studies blamed a past collision for such oddities, they disagreed over when the collision occurred, the mass of the satellite galaxy and whether the event involved a single large dwarf galaxy or several smaller ones. Helmi’s study, on the other hand, brings multiple lines of evidence together to paint the most compelling portrait yet, says Kathryn Johnston, an astronomer at Columbia University who was not involved in the work. And it is one that relies on a single—yet massive—merger.
Helmi and her colleagues analyzed 50,000 Gaia stars within the Milky Way’s halo, finding 33,000 of them share similar amounts of angular momentum and orbit in the opposite direction than they should. “That’s when it became apparent that this was weird,” she says. “The disk is ordered; you have 100 million stars moving orderly around the galactic center. And then you have these stars that have decided to move in the opposite sense. That hints that these stars could not have been formed in the Milky Way.” To further test that hypothesis, the team also analyzed the chemical compositions of 600 of those stars previously studied with the ground-based APOGEE stellar survey. In every galaxy the abundance of elements heavier than hydrogen and helium gradually increases over time due to cycles of stellar death and rebirth. But the exact pattern of that increase is specific to each galaxy, much like a fingerprint. Taken together with simulations modeling the potential merger, the Gaia and APOGEE data reinforce the notion these stars are truly alien interlopers, suggesting they originated within a single large dwarf galaxy that ceased forming stars 10 billion years ago—at the time it was cannibalized.
The team named the deceased galaxy “Gaia–Enceladus,” in honor of the space observatory and a being from Greek mythology, Enceladus, a giant who was believed to be buried under the Mount Etna volcano and responsible for the earthquakes in the region.
The study, if correct, firstly confirms what theorists have long thought: that galaxies like the Milky Way grow to enormous proportions by devouring many smaller ones. “It wasn’t known whether the Milky Way had experienced any mergers,” Helmi says. “And you never know when you see a merger in another galaxy, if it’s just anecdotal. So, the fact that we now have the data, and it does tell us that mergers have happened and have had a significant impact in the history of our galaxy—I think that’s a very important step in confirming this picture that we have that galaxies do build up via mergers.” The sheer confirmation alone has sent many astronomers into a state of euphoria.
To boot, the study also presents an unprecedented opportunity for new avenues of research. Although astronomers have spotted galaxies merging in distant corners of the cosmos, a collision seen within the Milky Way—even just a remnant—provides a front-row seat that offers far more answers. Even our nearest neighbor, Andromeda, is too far off for robust studies of mergers there. “At nearly three million light-years away, we’re never going to Andromeda to populate it or study it in detail,” says Kim Venn, an astronomer at the University of Victoria in British Columbia who was not involved in the study. “This is our only chance.” And with that chance, astronomers might better analyze the physics at play. Take the Milky Way’s spiral disk of stars as an example. That disk is composed of two parts: a thinner, dense region encompassed by a thicker, more diffuse one—but no one knows how the thick disk arose. Although Helmi notes the collision in question heated and puffed part of the thin disk into a thick one, future simulations of the collision will help astronomers better answer that question.
And the answer will only further shed light on these galactic wrecks. Indeed, Helmi wants to use the Milky Way’s evolution “as if it were a Rosetta stone”—to better understand how galaxies across the universe evolve and how they affect the cosmos. The results might even help astronomers fathom a future collision close to home, Venn says, when the Andromeda galaxy smacks directly into the Milky Way some four billion years from now.