Proof that the Milky Way experienced a significant merger only 1.5 billion years after the Big Bang

Abstract: The merger history of the Galaxy has been traced back firmly to redshift 2 (10 Billion years ago). While there have been claims of the existence of at least one more significant merger before this time, supporting evidence has been indirect and contentious. Here we show that the population of globular clusters around the Galaxy depicts three distinct age-metallicity sequences, one associated with the merger with Gaia-Enceladus 10 billion years ago, one to the progenitor of the Milky Way and a third intermediate sequence associated to at least one merger which we estimate took place merely 1.5 billion years after the Big Bang. 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 stellar mass similar to that of Gaia-Enceladus (~5x10$^8$ M$_{\odot}$), and which deposited most of its mass in the inner 6 kpc of the Milky Way. The unambiguous identification of a third merger event in the inner Galaxy puts to rest earlier debates, and honoring previous work we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.