Reprinted with permission from Quanta's Abstractions blog.
As the Milky Way was growing, taking shape, and minding its own business around 10 billion years ago, it suffered a massive head-on collision with another, smaller galaxy. That cosmic cataclysm changed the Milky Way's structure forever, shaping the thick spirals that spin out from the supermassive black hole at the galaxy's core. Two new studies—one published last month, another still under peer review—describe the evidence for this previously unnoticed event.
To uncover evidence of the collision so many eons later, astronomers have to work like galactic archaeologists, sifting through myriad sources of surviving information to piece together a story consistent with the available evidence. Both research teams relied on data from the European Space Agency's Gaia space telescope, which has spent years gathering exceptionally rich biographies of millions of stars—not only their locations and motions, but for many, their brightnesses, temperatures, ages, and composition as well. They essentially created high-resolution and multidimensional maps of the Milky Way and used these maps to find anomalous populations of old stars that appear to retain a memory of the long-ago collision. "The Gaia results really are allowing us to see things in the galaxy that we maybe suspected were there but haven't seen," said Kathryn Johnston, an astrophysicist at Columbia University.
He and his team found a large number of stars that aren't moving in step with the galaxy's rotation. Instead, they move in radial orbits, streaming toward or away from the center of the galaxy. These stars are also rich in "metals"—the catch-all description astronomers give to any element heavier than hydrogen, helium, or lithium. Metal-rich stars likely descend from many previous generations of stars. They're the scions of stars from a long-ago galaxy that smacked into the Milky Way, their orbits still reflecting the odd trajectory of that cosmic agitator.
Belokurov's group also modeled different collision scenarios, as well as a possible quieter history without significant collisions. An impact of a small "dwarf" galaxy indeed could have deposited a cloud of stars like the ones seen today, they found. Their work was published online last month in the Monthly Notices of the Royal Astronomical Society.
Ramin Skibba is an astrophysicist turned science writer and freelance journalist. His work has also appeared in Newsweek, Slate, Nature, Science, Scientific American, and New Scientist.