The thick disk rotation-metallicity correlation as a fossil of an
  "inverse chemical gradient" in the early Galaxy
  The thick disk rotation--metallicity correlation, \partial
V_\phi/\partial[Fe/H] =40\div 50 km s^{-1}dex^{-1} represents an important
signature of the formation processes of the galactic disk. 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. 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. Essentially, inner disk stars move towards the outer
regions and populate layers located at higher |z|. 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 \partial V_\phi/\partial[Fe/H]\simeq 60 km s^{-1}dex^{-1} for
particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Our pure N-body
models can account for the V_\phi 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.
