Chemical Cartography With Apogee: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk

Using a sample of 69,919 red giants from the SDSS-III/APOGEE Data Release 12, we measure the distribution of stars in the [ α /Fe] versus [Fe/H] plane and the metallicity distribution functions (MDFs) across an unprecedented volume of the Milky Way disk, with radius 3 < R < 15 kpc and height kpc. Stars in the inner disk ( R < 5 kpc) lie along a single track in [ α /Fe] versus [Fe/H], starting with α -enhanced, metal-poor stars and ending at [ α /Fe] ∼ 0 and [Fe/H] ∼ +0.4. At larger radii we find two distinct sequences in [ α /Fe] versus [Fe/H] space, with a roughly solar- α sequence that spans a decade in metallicity and a high- α sequence that merges with the low- α sequence at super-solar [Fe/H]. The location of the high- α sequence is nearly constant across the disk; however, there are very few high- α stars at R > 11 kpc. The peak of the midplane MDF shifts to lower metallicity at larger R , reflecting the Galactic metallicity gradient. Most strikingly, the shape of the midplane MDF changes systematically with radius, from a negatively skewed distribution at 3 < R < 7 kpc, to a roughly Gaussian distribution at the solar annulus, to a positively skewed shape in the outer Galaxy. For stars with kpc or [ α /Fe] > 0.18, the MDF shows little dependence on R . The positive skewness of the outer-disk MDF may be a signature of radial migration; we show that blurring of stellar populations by orbital eccentricities is not enough to explain the reversal of MDF shape, but a simple model of radial migration can do so.