Roughly 10 billion years ago, the Milky Way collided with another galaxy and devoured it whole.
But that second galaxy, called Gaia-Enceladus, didn’t just disappear. Even today, traces of this cataclysmic encounter remain speckled throughout our own galaxy, like the fragmented remains of a partially digested meal. And now, scientists studying some of the most crucial crumbs may be getting a glimpse of the Milky Way’s mysterious origins.
As researchers report in a study published today in the journal Nature Astronomy, many of the stars that populate the Milky Way today are relics from times long gone. Like a stellar fossil record, these ancient groups of stars demarcate crucial points in the Milky Way’s history, including its momentous merger with Gaia-Enceladus. Some stars may even be as old as the Milky Way itself, stretching some 13 billion years back in time.
This new examination of some of the Milky Way’s oldest stellar contents constitutes a “window into our galaxy’s past,” Chris Hayes, an astronomer at the University of Virginia who was not involved in the study, told Nadia Drake at National Geographic.
After being seeded by a series of star clusters, a growing galaxy will continue to gobble up matter from its surroundings, making it challenging to pinpoint where its “beginnings” actually end.
But aging a galaxy’s contents—namely, its stars—can at least help researchers construct a timeline of noteworthy events. Using data from the European Space Agency’s Gaia satellite, a team of astronomers led by Carme Gallart of the Institute of Astrophysics of the Canary Islands calculated the ages of almost 600,000 stars within about 6,500 light-years of Earth.
The team discovered that two clear categories of stars exist in the Milky Way’s halo—a sparsely-populated spherical region that entirely encompasses our galaxy’s main disk, which itself takes the shape of a flat, matter-packed pancake.
One of these populations was dominated by slightly older stars with a reddish hue that appear to have formed during the Milky Way’s earliest days. The other group of stars, however, was a bit younger and bluer, and seemed to stem from another source entirely: Gaia-Enceladus, when it smashed into the Milky Way some 3 billion years later.
Cowering before the massive Milky Way, the smaller, less populous Gaia-Enceladus must have looked like something of a stellar buffet—and our own galaxy wasted no time gobbling up its more diminutive neighbor.
But Gaia-Enceladus did not go gentle into that good night. The reason those native red stars are in the Milky Way’s halo is probably because they were booted out of the disk by the two galaxies’ raucous rendezvous. Even today, that’s where those ancient rose-tinted relics remain, interspersed among the ghosts of Gaia-Enceladus’ past.
From our galaxy’s perspective, this meetup was probably a memorable one—possibly the largest and most recent galactic merger in the Milky Way’s history, Gallart told Leah Crane at New Scientist.
What’s more, the foreign blue twinklers still retain some of their original flair, including a distinct chemical signature and pattern of motion—both traits that have helped astronomers pick these rogue stars out from the crowd, Drake reports.
Even for galaxies, you are what you eat—and then some.