  Recent analysis of the SDSS-III/APOGEE Data Release 12 stellar catalogue has
revealed that the Milky Way's metallicity distribution function (MDF) changes
shape as a function of radius, transitioning from being negatively skewed at
small Galactocentric radii to positively skewed at large Galactocentric radii.
Using a high resolution, N-body+SPH simulation, we show that the changing
skewness arises from radial migration - metal-rich stars form in the inner disk
and subsequently migrate to the metal-poorer outer disk. These migrated stars
represent a large fraction (> 50%) of the stars in the outer disk; they
populate the high metallicity tail of the MDFs and are, in general, more
metal-rich than the surrounding outer disk gas. The simulation also reproduces
another surprising APOGEE result: the spatially invariant high-[alpha/Fe] MDFs.
This arises in the simulation from the migration of a population formed within
a narrow range of radii (3.2+/-1.2 kpc) and time (8.8+/-0.6 Gyr ago), rather
than from spatially extended star formation in a homogeneous medium at early
times. These results point toward the crucial role radial migration has played
in shaping our Milky Way.
