Hubble finds Milky Way's earliest known galaxy merger

Astronomers using the Hubble Space Telescope have found evidence that the Milky Way devoured a smaller dwarf galaxy about 11.8 billion years ago, making it the earliest known merger in our galaxy's history. The discovery, published in Nature Astronomy, sheds light on how the Milky Way grew by absorbing smaller galaxies.

The discovery

Researchers led by Davide Massari of the National Institute for Astrophysics in Bologna studied 39 globular clusters in the inner 20,000 light-years of the Milky Way, combining Hubble's high-resolution imaging with data from the Gaia mission. They identified a distinct population of clusters that originated in a dwarf galaxy they named Low-energy-Kraken-Heracles (LKH). The merger occurred just 2 billion years after the Big Bang, when the universe was less than 15% of its current age.

Key findings

• The dwarf galaxy LKH had a stellar mass roughly 500 million times the Sun's, about a quarter of the Milky Way's size at the time. Source: SCMP
• The merger infused the Milky Way with stars, gas, and dark matter, potentially triggering a burst of star formation. Source: CNA
• Massari described the merger as "peaceful" for stars but "explosive" for gas, likely igniting new star formation. Source: CNA

Context

The Milky Way is known to have undergone two other major mergers: with Gaia-Sausage-Enceladus about 1.8 billion years after LKH, and with the Sagittarius dwarf galaxy over the past 6 billion years. Source: CNA The new finding fills a gap in our understanding of the galaxy's early evolution, which had previously been murky. Source: Gizmodo