Hubble's 'exquisite' data reveals one of the Milky Way's earliest mergers

Our home galaxy, the Milky Way, is a vast spiral structure teeming with billions of stars. However, it wasn't always this grand. Its growth has been a long process, fueled by the formation of new stars from gas clouds and the assimilation of smaller galaxies through cosmic mergers.

A team of astronomers, utilizing the exceptional data from the Hubble Space Telescope, has now pieced together crucial early chapters of the Milky Way's evolutionary story. Their research has uncovered compelling evidence of a significant merger event that took place around 11.8 billion years ago, a mere 2 billion years after the Big Bang.

The study, published in Nature Astronomy and led by Davide Massari of the Astrophysical Observatory of Bologna, Italy, analyzed 39 dense groupings of stars known as globular clusters. These ancient clusters serve as cosmic archaeological sites, offering clues to the galaxy's past. By combining Hubble observations with advanced statistical models, the scientists determined the ages and chemical compositions of these clusters.

This ancient merger, involving a dwarf galaxy known as "Gaia-Sausage-Enceladus" approximately 10 billion years ago, significantly impacted the structure of our galaxy's stellar disk. While previous simulations and observations had hinted at such a major fusion, the precise details remained a subject of debate. This latest work clarifies that a dwarf galaxy indeed merged with the nascent Milky Way during its formative stages.

The findings reveal a fundamental element that contributed to the formation of the young Milky Way around 12 billion years ago. This discovery offers a more precise chronology of the galaxy's initial expansion and structural development, answering long-standing questions about how our cosmic home was built.

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Our home is the Milky Way, but we don't know how our house was built.