Galactic archaeologists have uncovered a fascinating chapter in the Milky Way's history, revealing that our galaxy's formation was a complex process involving mergers with other galaxies. A recent study, published in Nature Astronomy, sheds light on one of the earliest significant events in the Milky Way's evolution, dating back 12 billion years. This research, led by astronomer Davide Massari from the Astrophysics and Space Science Observatory of Bologna, Italy, focuses on a merger that occurred around 1.8 billion years before another well-known event, Gaia–Sausage–Enceladus (GSE).
The study's significance lies in its ability to pinpoint the timing and characteristics of this ancient merger. By utilizing globular clusters, dense groups of stars formed simultaneously, as precise cosmic clocks, the team determined the relative ages of these clusters across the Milky Way. They then compared these ages with the clusters' metallicity, the abundance of elements heavier than hydrogen and helium, to understand the chemical evolution of the merging galaxies.
The results are striking. The Milky Way's globular clusters exhibit three distinct age-metallicity sequences. One sequence is associated with the early Milky Way, another with GSE, and the third with the earlier merger event. This earlier merger involved a galaxy with a stellar mass similar to GSE, containing approximately 500 million Suns' worth of stars. Interestingly, much of its material was deposited in the innermost parts of the Milky Way, connecting several previously named structures, which the authors collectively refer to as Low-energy–Kraken–Heracles (LKH).
What makes this discovery particularly intriguing is the level of precision it offers. While earlier studies had hinted at this early accretion event, the new research provides a more detailed timeline and description. By separating the chemical histories of the early Milky Way, LKH, and GSE, scientists can now explore the characteristics and evolutionary paths of these galaxies during their formative years.
This research highlights the importance of galactic archaeology, a field that combines astronomy and paleontology to study the fossil record of galaxies. By examining surviving stars and clusters from the early universe, astronomers can piece together the history of galaxy formation. The Milky Way, in particular, provides a unique perspective on the distant past, offering a detailed account of its formation and growth. As telescopes like the James Webb Space Telescope continue to observe distant galaxies, galactic archaeology plays a crucial role in interpreting these observations and constructing a comprehensive history of the universe's early galaxies.
