Chronology of our Galaxy from Gaia CMD-fitting (ChronoGal): the early formation of the Milky Way disk and the impact of Gaia-Sausage-Enceladus (2503.17304v1)

Abstract: The derivation of precise stellar ages is considered the current major challenge to reconstruct the chronology of the Milky Way. Color-magnitude diagram (CMD)-fitting offers a robust alternative to individual age determinations via the derivation of dynamically evolved star formation histories (deSFH) and age-metallicity distributions (Gallart et al. 2024). Our new suite of routines, CMDft.Gaia, specifically developed to analyse Gaia CMDs, produce deSFHs which are robust against sensible changes in the input parameters and extremely precise, providing an unprecedentedly detailed characterization of the successive events of star formation that, since its early evolution, have shaped the current Milky Way. Also important is the fact that, thanks to the high completeness of the Gaia photometric data, CMDft.Gaia provides the actual number of stars and the mass involved in the different events of star formation. The current analysis of the deSFH for stellar populations within 100 pc of the Sun, as well as for kinematically selected stars in the thin disk, thick disk, and halo, allows us to sketch a tentative picture of Milky Way evolution. The findings indicate that star formation commenced very early in a thick disk, with a small fraction of stars having [M/H]<-0.5 forming more than 12 Gyr ago. This phase culminated in a more prominent 12 Gyr old population with [M/H]~-0.5. Approximately 11 Gyr ago, the merger with GSE triggered an intense burst of star formation, generating most of the thick disk mass and enriching its metallicity to solar levels. Subsequently, the bulk of the star formation in the thin disk started and continues with a somewhat episodic behaviour up to the present time.