The team analysed Hubble observations of 39 globular clusters in the inner 20 000 light-years of our galaxy, where evidence of the most ancient mergers should be preserved. They expected this sample to contain globular clusters that formed within the young Milky Way galaxy as well as those collected from the Gaia-Sausage-Enceladus dwarf galaxy about 10 billion years ago.
Using Hubble’s sensitive observations to determine each cluster’s precise age and associated metallicity - the abundance of elements heavier than helium - they determined there was a third population of globular clusters in the inner regions of our galaxy. The team found that these clusters are older than the group collected in the Gaia-Sausage-Enceladus merger, but younger than those born in the Milky Way galaxy, regardless of their metal content. These clusters, therefore, came from a separate and even earlier merger - in which the Milky Way galaxy absorbed a dwarf galaxy containing roughly 500 million times the mass of the Sun in stars, a significant fraction of our galaxy’s mass at the time.
They named this dwarf galaxy Low-energy-Kraken-Heracles, or LKH, in honour of three earlier research papers that championed the idea of a merger early in our galaxy’s history. Such a large merger so early in the Milky Way galaxy’s formation has profound implications for the evolution of our galaxy.
“Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” says Davide. “Here, we have shown that stars born in external galaxies also need to be considered.”
The team plans to continue their work to unravel the history of the Milky Way galaxy by studying its globular clusters, aiming to characterise all the massive mergers that our galaxy has experienced across cosmic history.
“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, University of Vigo and the University of La Laguna in Spain.