  Radial migration is defined as the change in guiding centre radius of stars
and gas caused by gains or losses of angular momentum that result from
gravitational interaction with non-axisymmetric structure. This has been shown
to have significant impact on the metallicity distribution in galactic discs,
and therefore affects the interpretation of Galactic archeology. We use a
simulation of a Milky Way-sized galaxy to examine the effect of radial
migration on the star and gas radial metallicity distribution. We find that
both the star and gas component show significant radial migration. The stellar
radial metallicity gradient remains almost unchanged but the radial metallicity
distribution of the stars is broadened to produce a greater dispersion at all
radii. However, the metallicity dispersion of the gas remains narrow. We find
that the main drivers of the gas metallicity distribution evolution are metal
enrichment and mixing: more efficient metal enrichment in the inner region
maintains a negative slope in the radial metallicity distribution, and the
metal mixing ensures the tight relationship of the gas metallicity with the
radius. The metallicity distribution function reproduces the trend in the
age-metallicity relation found from observations for stars younger than 1.0 Gyr
in the Milky Way.
