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A recent study has revealed intriguing evidence suggesting that a long-lost dwarf galaxy, referred to as Loki, may have amalgamated with the early Milky Way. This finding sheds light on the violent and tumultuous formation history of our galaxy.
The Large Magellanic Cloud (LMC), a dwarf galaxy located nearby, possesses a mass equivalent to about 1 percent of the Milky Way. Loki, potentially double the size of the LMC, is proposed to have merged with the Milky Way during its formative years, according to a new analysis.
The Milky Way galaxy is characterized by a complex history marked by significant mergers rather than smooth accretion. A team of astronomers has identified remnants from what could be the most ancient of these merger events, situated close to our solar system.
Stars within the Milky Way exhibit a diverse range of ages, sizes, and metal abundances—elements heavier than hydrogen or helium. Stars born from the same primordial gas cloud typically share similar properties, yet they disperse into individual orbits as they age. Astronomers utilize variations in metal abundances to identify distinct star populations, including those that originated from external galaxies during merger events.
In research published on the arXiv preprint server, astronomers investigated a peculiar cluster of 20 stars in close proximity to the Sun. Contrary to typical solar stars, these exhibit minimal metal content, suggesting they are extremely ancient; the scarcity of heavy elements reflects a lack of multiple stellar generations necessary for their formation.
Despite their limited metal content, the stars maintain a consistent ratio of these elements, implying a shared origin. However, an interesting divergence is noted: while half of the stars orbit the Milky Way in one direction, the other half moves in the opposite direction.
Investigative Insights
Equipped with crucial data, researchers are piecing together the narrative of these stars. The considerable age disparity compared to typical disk stars indicates that these celestial bodies likely did not form within the Milky Way. Simulations concerning element distribution within galaxies suggest their original home was a small dwarf galaxy, with a mass around 2 percent of the Milky Way.
Multiple cosmic phenomena are responsible for generating heavy elements, including supernova explosions of massive stars and the merging of neutron stars. The specific arrangement of heavy elements found in this group of stars indicates they were enriched by massive star explosions and neutron star mergers, but not by the detonation of white dwarfs. Such findings imply that Loki underwent a brief and intense period of star formation, resulting in many stars dying before white dwarfs could explode.
The researchers have named this ancient galaxy Loki, after the Norse trickster god. Although it remains challenging to determine the exact timeline of Loki’s merger with the Milky Way, there is a significant clue: the differing orbital directions of the stars. This variability suggests that the merger occurred early in the Milky Way’s history, prior to the establishment of its current disk structure, allowing Loki’s stars to disperse chaotically upon integration.
Given that the Milky Way began forming roughly 10–11 billion years ago, Loki could very well represent one of the earliest mergers experienced by our galaxy. Consequently, this unique group of stars could be among the oldest remnants still residing in the Milky Way.
This investigative work not only illuminates the story of Loki but also contributes to the broader understanding of the various mergers that shaped the Milky Way into its current form.
Source
www.astronomy.com