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 indicated by recent research highlighting a significant event in its early history. Scientists studying star clusters near the Milky Way’s core have found evidence suggesting that approximately 11.8 billion years ago, when the universe was young, our galaxy engulfed a smaller galaxy. This event marks the earliest known galactic merger for the Milky Way, showcasing its growth through not only internal star formation but also the absorption of smaller galaxies within its gravitational influence.
The smaller galaxy involved in this merger was a dwarf galaxy, distinguishing it from larger galactic structures like the Milky Way. Researchers estimate that this dwarf galaxy contained stars with a combined mass equal to 500 million times that of the sun at the time of the merger, roughly a quarter of the Milky Way’s size back then. Utilizing the Hubble and Gaia telescopes, scientists examined stellar clusters within the innermost 20,000 light-years of the Milky Way, analyzing their age, composition, and trajectory.
These stars became part of the Milky Way following a galactic merger around two billion years post the Big Bang event that marked the universe’s inception. The originating dwarf galaxy was named Low-energy-Kraken-Heracles (LKH) in reference to previous studies exploring the likelihood of such a primordial merger.
Lead author Davide Massari, an astrophysicist from the National Institute for Astrophysics in Bologna, Italy, emphasized the groundbreaking discovery of the Milky Way’s first merger event during its early stages. The study sheds light on the debate around whether the Milky Way’s initial growth stemmed from internal star formation or galactic mergers. The findings suggest that the merger significantly contributed to the Milky Way’s early development by introducing a substantial amount of stars, interstellar gas supporting star formation, and dark matter.
Massari noted that while the merger was relatively peaceful in terms of stellar collisions, it triggered an “explosive” interaction between gases from the two galaxies, likely fueling the formation of numerous stars. The Milky Way has been involved in two other significant galactic mergers, including the absorption of 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, underlining the importance of understanding the Milky Way’s history in addressing fundamental questions about our origins.