Research on the formation, origin, and evolution of the dichotomy between the thin and thick disk components of the Milky Way has been a major topic of study, as it is key to understanding how our Galaxy formed. However, this is not an easy task, since populations defined by their morphology or kinematics show a mixture of chemically distinct stellar populations. Age therefore becomes a fundamental parameter for understanding the evolution of the Galactic disk. Our goal is to derive the age and metallicity distributions of the thin and thick disks defined kinematically, in order to reveal details about the duration, intensity, and relationship between the star formation episodes that led to the present-day kinematic structure of the thick and thin disks. We apply the CMDft.Gaia pipeline developed by our group, based on a color-magnitude diagram fitting technique, to derive the dynamically evolved star formation history (that is, affected by stellar migration within the disk) of the kinematically selected thin and thick disks. The analysis is based on Gaia DR3 data within a cylindrical volume centered on the Sun, with a radius of 250 pc and a height of 1 kpc.
Our results show that the kinematically selected thick disk is predominantly older than 10 Gyr and experienced rapid metallicity enrichment through three main star formation episodes. The first occurred more than 12 Gyr ago, peaking at a metallicity of [Z/H] ≃ −0.5 dex, or approximately one third of the solar metallicity. The second took place around 11 Gyr ago and caused a rapid increase in metallicity up to [Z/H] = 0.0 (solar metallicity), together with a wide spread in [α/Fe] from about 0.3 down to solar values. The third, slightly more than 10 Gyr ago, reached super-solar metallicities. In contrast, stars belonging to the kinematically defined thin disk began to form about 10 Gyr ago, coinciding with the end of thick-disk star formation. These stars are characterized by super-solar metallicity and low [α/Fe] values. The transition between the kinematically defined thick and thin disks coincides with the last major merger experienced by the Milky Way: the accretion of the Gaia-Sausage-Enceladus satellite galaxy.
We also identify a small population of kinematically selected thin-disk stars with high and intermediate [α/Fe] abundances and ages slightly older than 10 billion years. This indicates a kinematic transition from the thick disk to the thin disk during the high- and intermediate-[α/Fe] phase of the Milky Way. The age-metallicity relation of the kinematically defined thin disk reveals overlapping star formation episodes at different metallicities, suggesting radial migration in the solar neighborhood, with the largest dispersion occurring around 6 Gyr ago. In addition, we detect an isolated thick-disk star formation event at solar metallicity about 6 Gyr ago, which coincides with estimates of the first pericentric passage of the Sagittarius satellite galaxy.
