New observations from the NASA/ESA Hubble Space Telescope, combined with data from ESA’s Gaia mission, have provided evidence of a dwarf galaxy merging with the young Milky Way approximately 11.8 billion years ago. The finding extends knowledge of the Milky Way’s history by about 1.8 billion years compared with previous research.
The Milky Way is now a massive spiral galaxy containing hundreds of billions of stars, but it grew over time through the formation of new stars as well as by acquiring stars, gas and dark matter from other galaxies through mergers.
One of the most recent major mergers involved the Sagittarius dwarf galaxy, which began merging with the Milky Way more than 6 billion years ago and is still ongoing. Researchers have also found evidence that the Milky Way consumed another dwarf galaxy, Gaia-Sausage-Enceladus, around 10 billion years ago. That merger significantly influenced the structure of the Milky Way’s stellar disc.
Observations and simulations have suggested that an even earlier major merger occurred before these events, although the details have remained debated. The new Hubble observations provide evidence for a merger that occurred approximately 11.8 billion years ago, around 2 billion years after the Big Bang.
“Our home is the Milky Way galaxy, but we do not know how our house was built,” said Davide Massari, lead author from the Astrophysics and Space Science Observatory of Bologna in Italy. “In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH.”
Globular Clusters as Cosmic Archaeological Sites
Astronomical surveys and precision observations from missions such as ESA’s Gaia have helped researchers reconstruct the history of the Milky Way. Looking further into the galaxy’s past becomes increasingly difficult because the young Milky Way was smaller, closer in size to other galaxies and more chaotic than it is today. Evidence of ancient mergers may also have been altered or erased over billions of years.
To investigate this history, researchers used Hubble to study globular clusters within the Milky Way. These large, approximately spherical collections can contain tens of thousands to several million stars. Because globular clusters include some of the oldest stars in the galaxy, they can preserve evidence of galaxies that were incorporated into the Milky Way.
“Thanks to the high resolution and depth of Hubble imaging, we could measure the age and the metal content of these clusters with unprecedented precision,” said Chiara Zerbinati, a study co-author from the University of Bologna. “Coupled with measurements from Gaia, this made it possible to distinguish a population of globular clusters that are different from the others. These are the clusters that were born in LKH, and they tell us when that galaxy was devoured by ours, and how massive it was.”
The team analysed Hubble observations of 39 globular clusters located within the inner 20,000 light-years of the Milky Way, where evidence of ancient mergers is expected to remain preserved. The researchers expected the sample to include clusters that formed within the young Milky Way as well as clusters acquired during the Gaia-Sausage-Enceladus merger approximately 10 billion years ago.
Using Hubble observations to determine the ages and metallicities of the clusters, where metallicity refers to the abundance of elements heavier than helium, the researchers identified a third population of globular clusters in the inner regions of the galaxy.
These clusters were found to be older than the group associated with the Gaia-Sausage-Enceladus merger but younger than clusters formed within the Milky Way itself, regardless of their metallicity. The researchers concluded that the clusters originated in a separate, earlier merger involving a dwarf galaxy that contained approximately 500 million times the mass of the Sun in stars.
The researchers named the dwarf galaxy Low-energy-Kraken-Heracles, or LKH, in recognition of three earlier research papers that proposed an early merger in the history of the Milky Way.
The researchers said that such a large merger during the early stages of the Milky Way’s formation has important implications for understanding the galaxy’s evolution.
“Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” said Massari. “Here, we have shown that stars born in external galaxies also need to be considered.”
The team plans to continue studying the Milky Way’s globular clusters to further reconstruct its history and identify the major mergers the galaxy has experienced over cosmic time.
“Hubble is observing globular clusters that have never been studied before, and this will help us characterise the merger events that are far back in time in the Milky Way galaxy’s history,” said Fernando Aguado-Agelet, co-author from the University of Vigo and the University of La Laguna in Spain.
