Numerical Simulation of a possible origin of the positive radial
  metallicity gradient of the thick disk
  We analyze the radial and vertical metallicity and [alpha/Fe] gradients of
the disk stars of a disk galaxy simulated in a fully cosmological setting with
the chemodynamical galaxy evolution code, GCD+. We study how the radial
abundance gradients vary as a function of height above the plane and find that
the metallicity ([alpha/Fe]) gradient becomes more positive (negative) with
increasing height, changing sign around 1.5 kpc above the plane. At the largest
vertical height (2 < |z| < 3 kpc), our simulated galaxy shows a positive radial
metallicity gradient. We find that the positive metallicity gradient is caused
by the age-metallicity and age-velocity dispersion relation, where the younger
stars have higher metallicity and lower velocity dispersion. Due to the
age-velocity dispersion relation, a greater fraction of younger stars reach |z|
> 2 kpc at the outer region, because of the lower gravitational restoring force
of the disk, i.e. flaring. As a result, the fraction of younger stars with
higher metallicity due to the age-metallicity relation becomes higher at the
outer radii, which makes the median metallicity higher at the outer radii.
Combining this result with the recently observed age-metallicity and
age-velocity dispersion relation for the Milky Way thick disk stars suggested
by Haywood et al. (2013), we argue that the observed (small) positive radial
metallicity gradient at large heights of the Milky Way disk stars can be
explained by the flaring of the younger thick and/or thin disk stars.
