The Milky Way, a vast spiral galaxy housing billions of stars, including our sun, has undergone a complex journey to reach its current size, as revealed by recent research highlighting a significant event in its early history.
Scientists studying star clusters near the center of the Milky Way have found evidence indicating that approximately 11.8 billion years ago, when the universe was young, our galaxy absorbed a smaller galaxy. This marks the earliest known galactic merger for the Milky Way, demonstrating its growth through not only internal star formation but also by merging with smaller galaxies captured in its gravitational field.
The assimilated galaxy was a dwarf galaxy, distinguished from larger galaxies like the Milky Way. Researchers estimate it contained stars equivalent to 500 million solar masses at the time of the merger, about a quarter of the Milky Way’s size then.
By analyzing stellar clusters within the Milky Way’s innermost 20,000 light-years using the Hubble and Gaia telescopes, researchers determined their age, chemical composition, and trajectory. A light-year represents the distance light travels in a year, approximately 9.5 trillion kilometers.
These stars became part of the Milky Way following a galactic merger occurring about two billion years post the Big Bang’s inception. The dwarf galaxy of origin was named Low-energy-Kraken-Heracles (LKH) based on earlier studies exploring this primordial merger concept.
Lead author of the study published in Nature Astronomy, astrophysicist Davide Massari from the National Institute for Astrophysics in Bologna, Italy, stated that the research’s key finding was the identification of the galaxy’s first merger during its early stages.
The debate over whether the Milky Way’s initial growth stemmed from internal star formation or galactic mergers has persisted. The recent study indicates that the merger provided the Milky Way with a substantial volume of stars, interstellar gas fueling star formation, and dark matter in its early history.
Massari noted that the merger, from a stellar perspective, was relatively peaceful, with no star-to-star collisions. However, regarding gas dynamics, the merger was more explosive, likely triggering the formation of numerous stars through the collision of gas from LKH and the Milky Way.
Massari suggested that many stars originally formed in the dwarf galaxy could still be present within the Milky Way’s inner regions, possibly within stellar clusters. However, due to the abundance of interstellar dust in this area, identifying individual stars proves challenging.
The Milky Way has been involved in two other significant galactic mergers. It absorbed the Gaia-Sausage-Enceladus dwarf galaxy roughly 1.8 billion years after the LKH merger. Over the last six billion years, it has been merging with the Sagittarius dwarf galaxy.
Each merger has influenced events leading to the formation of our solar system, emphasizing the importance of reconstructing the Milky Way’s history in understanding our cosmic origins.