Astronomers have found stars dating from a long-ago collision between the Milky Way and another galaxy. The crash helps to explain why the Milky Way looks the way it does.
Quanta Magazine said: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 earlier this month, another still under peer review — describe the evidence for this previously unnoticed event.
“This is a big step forward,” said Elena D’Onghia, an astrophysicist at the University of Wisconsin who is unaffiliated with the new research. “It’s interesting because we can finally see what the history of the Milky Way is.”
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.
Hints of a dramatic collision had been seen before, but the indications had been inconclusive. A distinct clump of unique stars would have been a giveaway that they’re interlopers from elsewhere, but no such evidence exists. The long-ago collision so thoroughly shook things up that the telltale stars have been strewn throughout the galaxy. “There’s debris everywhere,” said Vasily Belokurov, an astronomer at the University of Cambridge and a leader of one of the two teams. “You’re basically surrounded by that debris now.”
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.
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