The Milky Way Swallowed a Dwarf Galaxy 11.8 Billion Years Ago, Its Earliest Known Merger

Hubble and Gaia data reveal the Milky Way absorbed a dwarf galaxy named Low-energy-Kraken-Heracles about 2 billion years after the Big Bang — the earliest merger in our galaxy's history.

Astronomers have pinned down the Milky Way's earliest-known act of galactic cannibalism: roughly 11.8 billion years ago, our galaxy swallowed a dwarf galaxy called Low-energy-Kraken-Heracles, or LKH for short.

Using the Hubble and Gaia space telescopes, researchers studied clusters of stars near the Milky Way's center — each containing hundreds of thousands of stars — and measured their age, chemical makeup, and motion. The data showed that a large chunk of the inner galaxy arrived about 2 billion years after the Big Bang, when the universe was less than 15% of its current age. The findings were published in the journal Nature Astronomy.

The dwarf galaxy, named LKH in recognition of three earlier studies, is estimated to have held stars with a combined mass roughly 500 million times that of the Sun — about a quarter the size of the Milky Way at the time.

"From the point of view of stars, the merger was rather peaceful, with no star-to-star crashes," said lead author Davide Massari of Italy's National Institute for Astrophysics. "But 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."

That collision seeded our galaxy with stars, interstellar gas, and dark matter — the raw material for everything that followed. "Every single one of these mergers had an influence on the events that have led to the formation of our own solar system, and ultimately of Earth," Massari said. "Reconstructing the pieces of the Milky Way's past effectively contributes to answering the big question about, 'Where do we come from?'"

Read the full report from SRN News and Geo News.