The thick disk rotation-metallicity correlation as a fossil of an “inverse chemical gradient” in the early Galaxy

Context. The thick disk rotation-metallicity correlation, ∂Vφ/∂[Fe/H] = 40 ÷ 50 km s-1 dex-1 represents an important signature of the formation processes of the galactic disk. Aims: We use nondissipative numerical simulations to follow the evolution of a Milky Way (MW)-like disk to verify if secular dynamical processes can account for this correlation in the old thick disk stellar population. Methods: We followed the evolution of an ancient disk population represented by 10 million particles whose chemical abundances were assigned by assuming a cosmologically plausible radial metallicity gradient with lower metallicity in the inner regions, as expected for the 10-Gyr-old MW. The two cases of a disk with and without a bar were simulated to compare the evolution of their kinematics and radial chemical properties. Results: Migration processes act in both cases and appear to be enhanced in the presence of a central bar. Essentially, inner disk stars move towards the outer regions and populate layers located at higher |z|. In the case of an evolved barred disk, a rotation-metallicity correlation appears, which well resembles the behaviour observed in our Galaxy at a galactocentric distance between 8 kpc and 10 kpc. In particular, we measure a correlation of ∂Vφ/∂ [Fe/H] ≃ 60 km s-1 dex-1 for particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Conclusions: Our pure N-body models can account for the Vφ vs. [Fe/H] correlation observed in the thick disk of our Galaxy, suggesting that processes internal to the disk such as heating and radial migration play a role in the formation of this old stellar component. In this scenario, the positive rotation-metallicity correlation of the old thick disk population would represent the relic signature of an ancient inverse chemical (radial) gradient in the inner Galaxy, which resulted from accretion of primordial gas.