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 \pm 1.2 kpc ) and time ( 8.8 \pm 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 .