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Astronomers have uncovered the 12.3-billion-year-old ruins of an ancient galaxy hiding in the jumbled jungle of the Milky Way’s center.
For billions of years, our home galaxy grew into a cosmic giant by crashing into and absorbing smaller neighboring dwarf galaxies. Over years of research, scientists have mapped several of these galactic mergers, including a slow-speed collision with the Sagittarius dwarf galaxy, which began roughly 6 billion years ago. Scientists say it continues today.
The discovery of this even earlier space merger, published in Nature Astronomy, helps make sense of the tangled mess that is the center of the galaxy. And it shows that scientists can use the ages and ingredients of star clusters to retrace the steps of the Milky Way’s violent past.
“Our home is the Milky Way galaxy, but we do not know how our house was built,” said Davide Massari, lead author of the research, in a statement. “In this paper, we discover where the first significant batch of bricks came from.”
Before this new finding, astronomers considered a collision with a massive galaxy named Gaia-Sausage-Enceladus to be the oldest major impact. That wreck occurred about 10 billion years ago and contributed to the bulge in the Milky Way’s massive, thick disk of stars.
Given that billions of years have passed, peering back into earlier phases of galactic history posed a considerable challenge for the team. The inner core of the primitive Milky Way acted like a giant gravitational blender, mixing ancient stars together and jostling their prior orbits. Because of this chaos, astronomers have spent years arguing whether the oldest stars in the galactic center grew there natively or immigrated from somewhere else by way of collisions.
To try to settle this debate, a team of researchers turned to NASA‘s Hubble Space Telescope to target globular clusters, tightly packed balls of thousands to millions of ancient stars. These clusters are sort of like fossils from the early universe.
The scientists studied 39 clusters in the inner galaxy, compared their relative ages, chemical distributions of heavy elements, and orbits using advanced statistical tools.
Their analysis revealed that the clusters split into three distinct families rather than the expected two: One group belonged to native Milky Way stars, another belonged to the known Gaia-Sausage-Enceladus merger, and a third revealed an entirely unknown population.
Dubbed Low-energy-Kraken-Heracles or LKH, the galaxy would have slammed into our galaxy roughly 1.8 billion years before Gaia-Sausage-Enceladus. The newly identified system’s name is a mashup of three earlier research papers that proposed a merger happened earlier in the galaxy’s history.
“Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” said Massari, who works at the Astrophysics and Space Science Observatory of Bologna in Italy. “Here, we have shown that stars born in external galaxies also need to be considered.”
Credit: NASA
Weighing roughly 500 million times more than the sun, the LKH system dumped most of its stars, gas, and other material straight into the inner 20,000 light-year span of the Milky Way.
Advanced computer simulations support the team’s findings, matching the timelines and chemical patterns predicted by the Hubble cluster data.
Astronomers are eager for future space telescope missions to scan the galactic core even deeper to reveal more information about the Milky Way’s beginnings. The team plans to continue scouring globular clusters for evidence, said Fernando Aguado-Agelet, a coauthor from the universities of Vigo and La Laguna in Spain, in a statement.
“This will help us characterize the merger events that are far back in time in the Milky Way galaxy’s history,” he said.
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