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The thick disk rotation--metallicity correlation, \partialV_\phi/\partial[Fe/H] =40\div 50 km s^{-1}dex^{-1} represents an importantsignature of the formation processes of the galactic disk. We usenondissipative numerical simulations to follow the evolution of a Milky Way(MW)-like disk to verify if secular dynamical processes can account for thiscorrelation in the old thick disk stellar population. We followed the evolutionof an ancient disk population represented by 10 million particles whosechemical abundances were assigned by assuming a cosmologically plausible radialmetallicity gradient with lower metallicity in the inner regions, as expectedfor the 10-Gyr-old MW. Essentially, inner disk stars move towards the outerregions and populate layers located at higher |z|. A rotation--metallicitycorrelation appears, which well resembles the behaviour observed in our Galaxyat a galactocentric distance between 8 kpc and 10 kpc. In particular,we measurea correlation of \partial V_\phi/\partial[Fe/H]\simeq 60 km s^{-1}dex^{-1} forparticles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Our pure N-bodymodels can account for the V_\phi vs. [Fe/H] correlation observed in the thickdisk of our Galaxy, suggesting that processes internal to the disk such asheating and radial migration play a role in the formation of this old stellarcomponent. In this scenario, the positive rotation-metallicity correlation ofthe old thick disk population would represent the relic signature of an ancient"inverse" chemical (radial) gradient in the inner Galaxy, which resulted fromaccretion of primordial gas.

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