What specific event occurred around 10 billion years ago in the Milky Way?

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A Major Galactic Collision Shaped the Milky Way Approximately 10 Billion Years Ago

Around 10 billion years ago, the Milky Way experienced a significant cosmic event: the collision and subsequent fusion with a dwarf galaxy known as Gaia-Enceladus. This merger fundamentally altered the structure of our galaxy, contributing to the formation of its stellar halo and influencing its subsequent growth. The discovery challenges previous notions of the Milky Way's development, suggesting a history marked by dramatic mergers rather than gradual accretion.
The Merger with Gaia-Enceladus

The collision between the Milky Way and Gaia-Enceladus occurred approximately 10 billion years ago. This event was not a minor encounter but a substantial fusion that integrated the stars of the dwarf galaxy into the nascent Milky Way. These stars now form a significant component of the halo that surrounds our current galaxy.
Formation of the Stellar Halo and Disk

As a direct consequence of the Gaia-Enceladus merger, new stars were born within the combined galactic mass. This period of intense star formation, triggered by the collision, also led to the creation of a "puffed up" disk structure. This characteristic feature of the Milky Way is a direct result of this ancient galactic interaction.
Evidence from Galactic Archaeology

This understanding of the Milky Way's past is largely derived from the field of galactic archaeology, which uses stars as historical records. By analysing stellar properties, such as their chemical composition and motion, astronomers can infer their origins and the environments in which they were born. The European Space Agency's Gaia mission, which precisely mapped the positions and movements of billions of stars, provided crucial data for reconstructing this history.
Hierarchical Accretion Model

The findings related to the Gaia-Enceladus merger support the theory of hierarchical accretion in galaxy formation. This model posits that larger galaxies, like the Milky Way, are built up over cosmic time through the merging of smaller galaxies. The recognition of this violent, transformative event has earned astronomers like Amina Helmi prestigious awards, highlighting the importance of understanding our galaxy's tumultuous origins.

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What specific event occurred around 10 billion years ago in the Milky Way?

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