A recent study reveals that the Milky Way, a vast spiral galaxy housing billions of stars, underwent a significant event early in its history. Scientists studying star clusters near the galaxy’s core have found evidence of a smaller galaxy merging with the Milky Way approximately 11.8 billion years ago when the universe was young. This marks the earliest-known galactic merger for the Milky Way, showcasing its growth not only through internal star formation but also by assimilating smaller galaxies within its gravitational influence.
The merging galaxy was classified as a dwarf galaxy, containing stars with a mass equivalent to 500 million suns at the time of the merger, making it about a quarter the size of the Milky Way at that period. Researchers utilized the Hubble and Gaia telescopes to analyze stellar clusters within the Milky Way’s innermost 20,000 light-years, studying their age, composition, and trajectory. A light-year represents the distance light travels in one year, equivalent to 9.5 trillion kilometers.
Following the galactic merger that occurred about two billion years post the Big Bang, these stars became integrated into the Milky Way. The dwarf galaxy of origin was named Low-energy-Kraken-Heracles (LKH) based on previous studies exploring the possibility of such a merger.
Lead author Davide Massari, an astrophysicist at the National Institute for Astrophysics, emphasized the significance of the study’s findings, identifying it as the earliest merger event experienced by the Milky Way. The research sheds light on the debate regarding the Milky Way’s growth, indicating that the merger introduced a substantial volume of stars, interstellar gas fueling star formation, and dark matter into the galaxy.
Massari described the merger as “peaceful” in terms of star interactions but more “explosive” concerning gas dynamics, suggesting that the collision between gas from LKH and the Milky Way likely triggered significant star formation. The Milky Way has been involved in two other major galactic mergers, including one with the Gaia-Sausage-Enceladus dwarf galaxy and ongoing interactions with the Sagittarius dwarf galaxy over the past six billion years.
These mergers have influenced the evolution of our solar system and Earth, underscoring the importance of understanding the Milky Way’s history in addressing profound questions about our origins.