Astronomers have discovered that the Milky Way merged with a massive ancient dwarf galaxy billions of years ago [1, 2, 3].
This discovery identifies the earliest known major collision in the history of the galaxy. Understanding these ancient mergers allows researchers to trace how the Milky Way grew and evolved into its current spiral structure.
Researchers, including those from Durham University, found evidence of the "lost galaxy," also referred to as LKH [1, 2]. The merger involved a dwarf galaxy containing stars with a combined mass equivalent to hundreds of millions of Suns [4]. This massive influx of material contributed to the overall growth, and structural evolution, of the Milky Way [1, 3].
Estimates regarding the timing of the collision vary across reports. Some data suggest the merger occurred about 10 billion years ago [1], while other sources place the event at approximately 12 billion years ago [2]. One specific estimate dates the event to 11.8 billion years ago [3].
The process of devouring the smaller galaxy left lasting imprints on the Milky Way. By analyzing the movement and composition of ancient stars, astronomers can reconstruct the trajectory of the collision. This event represents a pivotal moment in the galaxy's youth—a period characterized by violent growth and the absorption of smaller neighbors.
Such mergers are common in the early universe, but the scale of this specific collision highlights the aggressive nature of the Milky Way's early development. The integration of the dwarf galaxy provided the raw materials necessary for the galaxy to expand its disc, and stabilize its rotation, over eons.
“The Milky Way merged with a massive ancient dwarf galaxy billions of years ago.”
The identification of the LKH merger provides a missing link in galactic archaeology. By confirming that the Milky Way absorbed a massive dwarf galaxy between 10 and 12 billion years ago, scientists can better model the distribution of dark matter and the formation of the galactic halo. This suggests that the Milky Way's current mass and shape are not the result of gradual accumulation, but of episodic, violent events.
