The thick disk rotation-metallicity correlation, comparison with Galactic cosmological simulations

Although the existence of a thick disc in the Milky Way was revealed 35 years ago and its spatial, kinematic, and chemical properties are today better defined, its origin is still matter of debate. Proposed scenarios include the heating of a pre-existing thin disc through a minor merger, accretion of dwarf galaxies stars from disrupted satellites, or stars formed in situ from gas-rich mergers at high redshift. In order to better understand these processes, we have investigated the chemo-dynamical evolution of a Milky Way-like disk galaxy, as produced by the recent cosmological simulations, integrating a sub-resolution ISM model, published by Murante et al. (2015). In particular, we evidence a global inside-out and top-down disk evolution. Recently, Re Fiorentin, Lattanzi & Spagna (2019) analysed a new chemo-kinematic catalogue based on Gaia DR2 and APOGEE DR14 and showed evidence that the thick disk rotation-metallicity correlation is persistently positive from R=5 kpc to 13 kpc, in spite of a quasi-flat metallicity gradient. Our simulation at redshift z=0 shows very similar properties when we look at the “thick disk” stellar particles at 1 kpc <|z| < 3 kpc from the plane and at 6 kpc < R < 8 kpc from the galactic center. Here we show that similar trends seen in the Milky Way could have resulted from of the chemo-dynamical evolution of the galactic disk, starting from a primordial disk (redshift z > 2) with a negative rotation-metallicity correlation associated with a negative radial metallicity gradient (cfr. also Schoenrich & McMillan 2017; Kawata et al. 2018)