Scientists have discovered that the Milky Way devoured a massive dwarf galaxy approximately 11.8 billion years ago, representing the earliest-known galactic merger in our home galaxy’s history. Published in The Conversation, the new research shows that the Milky Way grew not only through internal star formation, but also by pulling in and absorbing smaller galaxies via its gravitational force.
Milky Way Swallowed Massive Dwarf Galaxy in Infancy
The ancient collision took place about 2 billion years after the Big Bang, at a time when the universe was less than 15 per cent of its current age. Researchers estimate that the devoured dwarf galaxy contained stars with a mass equivalent to 500 million times that of the sun, making it roughly a quarter the size of the infant Milky Way at the time. Much of the incoming material was deposited directly into the innermost parts of the Milky Way.
Using Hubble and Gaia Telescopes to Decode Star Clusters
To reconstruct this formative period, the research team studied tight-knit groups of stars known as globular clusters, each containing hundreds of thousands of stars. Researchers utilized data from the orbiting ABC News & Headlines – Australian Broadcasting Corporation and the European Space Agency’s Gaia satellite to gauge the age, chemical composition, and trajectories of stars inhabiting the Milky Way’s innermost 20,000 light-years.
Lead author Davide Massari, an astrophysicist at the National Institute for Astrophysics (INAF) in Bologna, Italy, noted that the existence of the 12-billion-year-old merger had been hotly debated according to the Australian Broadcasting Corporation. By examining exceptionally precise relative ages of dense star clusters, the team connected several previously named structures—including Kraken, Heracles, and a low-energy group of globular clusters—into a single progenitor named Low-energy–Kraken–Heracles, or LKH.
Finding such a clear-cut signature in the age of globular clusters made us very happy,
Dr. Massari said.
A Peaceful Star Collision and an Explosive Gas Merger
While the merger brought a massive influx of stars, interstellar gas, and dark matter into the Milky Way, the physical process of integration varied depending on the material involved. According to Massari, the stellar merger was relatively peaceful without direct star-to-star crashes, but the interaction involving gas was far more dynamic.
From the point of view of the gas, the merger was more ‘explosive’ — likely, the collision between the gas from LKH and that from the Milky Way triggered the formation of many stars,
Massari explained.
Researchers suggest that specific globular clusters, such as NGC 6535 located 22,000 light-years away, originated within the LKH galaxy. Although identifying individual stars from the ancient dwarf galaxy is exceptionally difficult due to heavy interstellar dust in the Milky Way’s center, many such stars likely remain buried within the galaxy’s inner regions.
Context Within the Milky Way’s Violent Assembly History
The Milky Way has a long history of absorbing neighboring systems. Following the LKH merger, the Milky Way later absorbed another dwarf galaxy known as Gaia–Sausage–Enceladus (GSE) approximately 1.8 billion years afterward. Over the past 6 billion years, the Milky Way has also been combining with the Sagittarius dwarf galaxy.
Scientists emphasize that each of these ancient collisions played a role in shaping the environment that eventually permitted the formation of our solar system and Earth.
