What crucial event shaped the Milky Way around 10 billion years ago?

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A significant merger event approximately 10 billion years ago fundamentally shaped the Milky Way.

Around 10 billion years ago, the Milky Way galaxy underwent a cataclysmic merger with a dwarf galaxy known as Gaia-Enceladus. This collision was not a minor event but a transformative one, leading to the formation of a substantial halo surrounding our galaxy and triggering the birth of new stars. This discovery has significantly altered our understanding of galactic evolution, demonstrating that the Milky Way's structure is a product of numerous mergers rather than a process of gradual accretion.
Collision with Gaia-Enceladus

The pivotal event involved the dwarf galaxy Gaia-Enceladus, which merged with the nascent Milky Way approximately 10 billion years ago. This cosmic crash predates the formation of our Sun and the majority of stars currently visible in the night sky. The fusion of these two galactic bodies was a monumental occurrence in the galaxy's history.
Formation of Galactic Halo and Stellar Generation

The immediate consequence of the Gaia-Enceladus merger was the creation of a substantial halo encircling the Milky Way. Furthermore, the intense gravitational interactions and shockwaves generated by the collision spurred a significant wave of star formation. This burst of stellar birth contributed to the galaxy's subsequent evolution and structural development.
Restructuring of the Galactic Disk

This dramatic merger is also credited with reshaping the Milky Way's central disk. The influx of material and the gravitational disturbances caused the disk to become "puffed up," a characteristic feature of our galaxy. This structural change is a direct observable legacy of the ancient collision.
Evidence for Hierarchical Accretion

The discovery, spearheaded by astronomers like Amina Helmi, provides compelling evidence that the Milky Way was not built through slow, steady growth. Instead, it supports the theory of hierarchical accretion, where large galaxies like the Milky Way form through the accumulation and merging of smaller galaxies over cosmic timescales. This finding has profound implications for our understanding of galaxy formation across the universe.

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What crucial event shaped the Milky Way around 10 billion years ago?

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