News release

From: Springer Nature

Ancient merger may reshape Milky Way history

An important building block that helped to construct the young Milky Way Galaxy about 12 billion years ago is reported in Nature Astronomy. The findings appear to confirm a merger with a system named Low-energy-Kraken-Heracles that may help explain the origin of some stars in the inner Milky Way and provide a clearer timeline of the Galaxy’s early growth.

Galaxies like the Milky Way are thought to assemble over billions of years through mergers with smaller systems, such as dwarf galaxies. Although several such events have been identified in the Milky Way’s formation history — most notably the Gaia–Sausage-Enceladus merger around 10.5 billion years ago — the earliest evolutionary stages of the proto-Galaxy have been difficult to reconstruct.

Davide Massari and colleagues analysed 39 dense groups of stars in the Milky Way, called globular clusters, using observations from the Hubble Space Telescope and advanced statistical modelling to determine their ages and chemical compositions. They identified three events in the Milky Way’s history using a process known as age-metallicity sequencing. One sequence traces the proto-Galaxy itself, another corresponds to the Gaia–Sausage-Enceladus merger, and a third reveals a merger that occurred approximately 1.8 billion years before the Gaia–Sausage-Enceladus merger. This system, named Low-energy-Kraken-Heracles, is estimated to have had a stellar mass similar to the Gaia–Sausage-Enceladus dwarf galaxy and deposited most of its material into the inner regions of the Galaxy.

The authors note that smaller or less well-preserved merger events prior to the Gaia–Sausage-Enceladus may remain undetected, and that further data (including precise stellar ages for a large number of stars in the inner Galaxy) are needed to fully reconstruct the sequence of early Galactic assembly.