New research has shed light on the complex journey of the Milky Way galaxy, a vast spiral structure hosting billions of stars, including our sun. Scientists studying star clusters near the galaxy’s center have uncovered evidence of a significant event in its past – the consumption of a smaller galaxy approximately 11.8 billion years ago, when the universe was still young.
This early galactic merger, the earliest known for the Milky Way, reveals how the galaxy expanded not only through internal star formation but also by assimilating smaller galaxies affected by its gravitational pull. The smaller galaxy, classified as a dwarf galaxy, contained stars equivalent to 500 million solar masses at the time of the merger, about a quarter of the Milky Way’s size back then.
Using the Hubble and Gaia telescopes, researchers examined stellar clusters within the Milky Way’s innermost 20,000 light-years to determine their age, composition, and trajectory. These stars, part of the galactic merger that occurred around two billion years post-Big Bang, originated from a dwarf galaxy named Low-energy-Kraken-Heracles (LKH).
Astrophysicist Davide Massari, the lead author of the study published in Nature Astronomy, emphasized the peaceful nature of the Milky Way’s early growth resulting from the merger. This event introduced a significant influx of stars, interstellar gas supporting star formation, and dark matter to the Milky Way, shaping its evolution.
Massari highlighted the impact of past galactic mergers on the formation of our solar system, underscoring the importance of understanding the Milky Way’s history in addressing fundamental questions about our origins. The Milky Way has undergone two other major mergers, including the absorption of Gaia-Sausage-Enceladus and the ongoing interaction with the Sagittarius dwarf galaxy over the last six billion years.