Hubble Telescope Uncovers Ancient Cosmic Collision In Milky Way's Past

Astronomers using NASA's Hubble Space Telescope have unlocked a long-standing cosmic mystery, revealing definitive evidence of a massive galactic collision that took place nearly 12 billion years ago. According to NASA, the discovery pushes scientists' understanding of the Milky Way's growth 1.8 billion years further back into the past than previously documented. Details of the findings were published Monday in the journal Nature Astronomy.

As we know, our home galaxy today is a massive spiral galaxy with hundreds of billions of stars. But it wasn't always like that. Scientists have noted that the galaxy was once smaller, and it eventually grew by forming new stars from its gas clouds as well as collecting stars, gas, and dark matter from other galaxies through mergers.

In an official statement, NASA revealed that a young Milky Way absorbed a dwarf galaxy known as LKH (Low-energy-Kraken-Heracles) approximately 11.8 billion years ago, just 2 billion years after the Big Bang.

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"Our home is the Milky Way galaxy, but we do not know how our house was built," Davide Massari, lead author, Astrophysics and Space Science Observatory of Bologna in Italy, said as quoted in the statement. "In this paper, we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH."

Scientists used astronomical surveys and precision data from spacecraft like European Space Agency's Gaia mission to piece together the history of our galaxy. They analyzed 39 ancient globular clusters, located within 20,000 light-years of the Milky Way's core.

"Thanks to the high resolution and depth of Hubble imaging, we could measure the age and the metal content of these clusters with unprecedented precision," Chiara Zerbinati, study co-author, University of Bologna in Italy, said in the statement.

"Coupled with measurements from Gaia, this made it possible to distinguish a population of globular clusters that are different from the others. These are the clusters that were born in LKH, and they tell us when that galaxy was devoured by ours, and how massive it was."

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By examining the exact ages and heavy element content ("metallicity") of these clusters, scientists identified three separate groups, clusters native to the early Milky Way, clusters absorbed during a well-known merger with the Gaia-Sausage-Enceladus galaxy 10 billion years ago, a newly confirmed, distinct set of clusters born in LKH before it was consumed 11.8 billion years ago.

At the time of impact, LKH had a stellar mass equal to roughly 500 million Suns. Because the young Milky Way was much smaller back then, this merger contributed a major fraction of the stars, gas, and dark matter that formed our galaxy's foundation.

"Some past studies have argued that the earliest phases of our galaxy's evolution were defined by stars born only in our galaxy," Massari added. "Here, we have shown that stars born in external galaxies also need to be considered."