Scientists have uncovered a tumultuous past for our galaxy: A Roman road has 'swallowed up' its neighbor

Astronomers have uncovered evidence suggesting that the Milky Way galaxy, which contains our solar system and Earth, consumed a smaller neighboring galaxy approximately 11.8 billion years ago. This discovery reveals a more chaotic early history for the Milky Way than previously thought. The collision occurred roughly two billion years after the Big Bang, when the universe was less than 15 percent of its current age. The smaller galaxy, named Low-energy Kraken Heracles (LKH), had a combined stellar mass comparable to about 500 million suns. At the time of the merger, LKH was about one-quarter the size of the young Milky Way. The findings provide scientists with a deeper look into the formation history of the Milky Way. According to NASA, these new data push back the known history of mergers involving the Milky Way by approximately 1.8 billion years compared to previous reconstructions. Evidence of the consumed galaxy still exists within the Milky Way today. How can astronomers determine what happened nearly 12 billion years ago? The key lies in ancient star clusters located in the inner regions of the Milky Way. An international team of researchers led by astrophysicist Davide Massari used data from space telescopes such as Hubble and Gaia. They studied spherical star clusters within approximately 20,000 light-years of the Milky Way’s center and compared their ages, chemical compositions, and movements. Through this analysis, they identified a group of star clusters likely not formed within the Milky Way itself. These clusters are cosmic fossils of the former LKH galaxy, drawn in by the gravity of the young Milky Way millions of years ago. The research has been published in the scientific journal Nature Astronomy. The authors concluded that LKH deposited much of its mass into the inner regions of our galaxy. Contrary to the dramatic image of galaxies colliding with billions of stars crashing into each other, the merger was relatively calm in terms of individual stars. Massari explained to Reuters that the merger did not involve large-scale direct collisions between stars. Instead, the more turbulent activity involved massive amounts of gas. When gas from the smaller galaxy met the gas of the Milky Way, it could have triggered intense star formation. Alongside stars, LKH also brought interstellar gas and dark matter to the young Milky Way. The Milky Way has not only consumed LKH but also other galaxies over time. Approximately 1.8 billion years later, the Milky Way merged with another galaxy now known as Gaia Sausage Enceladus. Additionally, the Milky Way continues to merge with a dwarf galaxy in the constellation Sagittarius. Such events were crucial in shaping the Milky Way as we know it today. Researchers are piecing together the family tree of our galaxy through the identification of long-lost neighbors. Massari emphasized that these individual mergers influenced subsequent events that eventually led to the formation of our solar system and Earth.