Abundance gradients in spiral disks: is the gradient inversion at high
  redshift real?
  We compute the abundance gradients along the disk of the Milky Way by means
of the two-infall model: in particular, the gradients of oxygen and iron and
their temporal evolution. First, we explore the effects of several physical
processes which influence the formation and evolution of abundance gradients.
They are: i) the inside-out formation of the disk, ii) a threshold in the gas
density for star formation, iii) a variable star formation efficiency along the
disk, iv) radial flows and their speed, and v) different total surface mass
density (gas plus stars) distributions for the halo. We are able to reproduce
at best the present day gradients of oxygen and iron if we assume an inside-out
formation, no threshold gas density, a constant efficiency of star formation
along the disk and radial gas flows. It is particularly important the choice of
the velocity pattern for radial flows and the combination of this velocity
pattern with the surface mass density distribution in the halo. Having selected
the best model, we then explore the evolution of abundance gradients in time
and find that the gradients in general steepen in time and that at redshift z~3
there is a gradient inversion in the inner regions of the disk, in the sense
that at early epochs the oxygen abundance decreases toward the Galactic center.
This effect, which has been observed, is naturally produced by our models if an
inside-out formation of the disk and and a constant star formation efficiency
are assumed. The inversion is due to the fact that in the inside-out formation
a strong infall of primordial gas, contrasting chemical enrichment, is present
in the innermost disk regions at early times. The gradient inversion remains
also in the presence of radial flows, either with constant or variable speed in
time, and this is a new result.
