Evidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger

The merger history of the Galaxy has been traced back firmly to a redshift of 2 (10 billion years ago). While evidence for the existence of at least one more substantial merger before this time has been presented, its interpretation is yet to be fully established. Here we show that the population of globular clusters around the Galaxy depicts three distinct age–metallicity sequences, one associated with the progenitor of the Milky Way, one with the merger with Gaia-Sausage-Enceladus 10 billion years ago and a third intermediate sequence associated with at least one merger that we estimate took place about 1.8 billion years before Gaia-Sausage-Enceladus. This discovery has been possible thanks to exquisite Hubble Space Telescope data and sophisticated analysis that enables very precise relative age determination of globular clusters. The newly identified sequence reveals that this merger took place with an object of a similar stellar mass to that of Gaia-Sausage-Enceladus (≃5 × 10 8 stellar masses, M ⊙ ) that deposited most of its mass in the inner 6 kpc of the Milky Way. The identification of a third merger event in the inner Galaxy puts to rest earlier debates and, honouring previous works, we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.