New research has shed light on the complex history of the Milky Way, a vast spiral galaxy housing billions of stars, including our sun. Scientists have identified a significant event that took place in the Milky Way’s early days, revealing that approximately 11.8 billion years ago, our galaxy consumed a smaller galaxy. This galactic merger marked the earliest known merger for the Milky Way, showcasing its growth not only through internal star formation but also through assimilating smaller galaxies drawn into its gravitational field.
The smaller galaxy involved in the merger was categorized as a dwarf galaxy, distinguishing it from larger galaxies like the Milky Way. Researchers estimate that this dwarf galaxy contained stars equivalent to 500 million solar masses at the time of the merger, roughly a quarter of the Milky Way’s size back then.
Utilizing the Hubble and Gaia telescopes, scientists studied stellar clusters within the Milky Way’s innermost 20,000 light-years to analyze their age, chemical makeup, and movement. These stars became part of the Milky Way following a merger around two billion years post the Big Bang event that initiated the universe. The dwarf galaxy of origin was dubbed Low-energy-Kraken-Heracles (LKH), reflecting previous studies exploring the likelihood of such a primordial merger.
Lead author Davide Massari, an astrophysicist at the National Institute for Astrophysics in Bologna, Italy, emphasized the study’s key finding: the clear identification of the Milky Way’s initial merger event, providing insights into the galaxy’s early development.
Debates over whether the Milky Way’s growth was driven by internal star formation or galactic mergers have persisted. The recent study suggests that the merger injected a substantial amount of stars, interstellar gas, and dark matter into the Milky Way during its formative years, fueling star formation.
Massari described the merger as “peaceful” in terms of stellar interactions but “explosive” concerning gas dynamics, potentially triggering prolific star formation. The Milky Way has been involved in two other significant galactic mergers, including the absorption of Gaia-Sausage-Enceladus and the ongoing merger with the Sagittarius dwarf galaxy over the last six billion years.
These merger events have played a crucial role in shaping the Milky Way’s history and, ultimately, impacting the formation of our solar system and Earth. Understanding the Milky Way’s past is instrumental in unraveling the profound question of our cosmic origins.