Uncovering the birth of the Milky Way through accurate stellar ages with Gaia
Résumé
Accurate distances to individual Milky Way stars provided by Gaia have allowed us to derive stellar age distributions from colour-magnitude diagrams (CMD) representative of the thick disc and the local halo. The CMD for the kinematically selected halo stars showed an enigmatic double main sequence. The blue sequence had been linked to a major accretion event, Gaia-Enceladus, whereas debate existed about the nature of the redder stars. We showed that both halo sequences share identical old age distributions, owing their difference in colour to the higher metallicity of the stars in the red sequence. These age distributions, together with cosmological simulations of galaxy formation, allowed us to identify the red sequence stars as the first stars formed in our Galaxy (the long-sought in-situ halo), and date the accretion of Gaia-Enceladus ∼10 Gyr ago.