Hubble Discovers the Earliest Known Passage in Galactic History
The Hubble Space Telescope has recently unveiled the most ancient passage known in the history of the Milky Way. A dedicated team of astronomers has identified a collection of stars that once belonged to a substantial galaxy, which merged with the proto-Milky Way approximately 12 billion years ago. This discovery represents the earliest evidence of a significant merger identified to date, extending the known timeline of major galactic collisions by 1.8 billion years, to a point when less than 2 billion years had passed since the Big Bang.
Reconstructing the Milky Way’s Family Tree
The prevailing model of galaxy evolution suggests a hierarchical growth pattern. This implies that the earliest galactic structures were relatively small, gradually evolving to form new stars and merging with other systems over time. The Milky Way exemplifies this process, having assimilated numerous other galaxies over billions of years, with their stars and other components contributing to an increasingly large and complex system.
These remnants of ancient galaxies are ubiquitous; however, the challenge lies in the sheer number surrounding us. The further back astronomers look, the more difficult it becomes to discern their origins. For decades, scientists have been classifying nearby stars to ascertain which belonged to other galaxies and when these mergers occurred. The oldest recorded collision until now dated back 10 billion years, when the proto-Milky Way absorbed the Gaia-Sausage-Enceladus (GSE) galaxy.
While there were hints of even older collisions, their interpretations remained contentious. Now, as NASA remarks, a significant collection of the foundational ‘bricks’ of our galaxy has been identified. Thanks to Hubble’s capabilities, definitive evidence has surfaced of another merger that took place 1.8 billion years prior to the major collision with GSE.
Introducing the Ancient Galaxy LKH
The newly identified galaxy had already provided some clues. For years, astronomers noted three populations of stars and clusters at the centre of the Milky Way that appeared to originate from ancient mergers. These were termed Low-energy, Kraken, and Heracles. It was initially unclear whether they shared a common origin or if they had all arrived from different galaxies, but recent research asserts they are remnants of a single system, now referred to as Low-energy-Kraken-Heracles (LKH).
Chiara Zerbinati, a co-author of the study, stated in a NASA release, “Thanks to the high resolution and depth of Hubble’s images, we were able to measure the age and metallic content of these clusters with unprecedented accuracy. Alongside measurements from Gaia, this allowed us to distinguish a population of globular clusters that are different from the others. These are the clusters that originated in LKH, indicating when that galaxy was consumed by ours and its mass.”
Insights into the Mass and Impact of LKH
The calculations suggest that LKH had a stellar mass of approximately 500 million solar masses, akin to that of Gaia-Sausage-Enceladus, with much of its material finding its way into the inner regions of the Milky Way. The merger occurred at a time when the Milky Way was still in its youth and significantly smaller, implying that this fusion played a crucial role in its evolutionary trajectory. The researchers estimate that LKH’s mass was over one-fifth that of the system with which it merged.
There is a strong likelihood that even older collisions exist, and scientists are committed to utilising all available resources to delve deeper into the untold history of the Milky Way.