Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk

Abstract

Using a sample of 69,919 red giants from the SDSS-III/APOGEE Data Release 12, we measure the distribution of stars in the [$\hat{I}\pm$/Fe] vs. [Fe/H] plane and the metallicity distribution functions (MDF) across an unprecedented volume of the Milky Way disk, with radius $3<R<15$ kpc and height $|z|<2$ kpc. Stars in the inner disk ($R<5$ kpc) lie along a single track in [$\hat{I}\pm$/Fe] vs. [Fe/H], starting with $\hat{I}\pm$-enhanced, metal-poor stars and ending at [$\hat{I}\pm$/Fe]$\sim 0$ and [Fe/H]$\sim +0.4$. At larger radii we find two distinct sequences in [$\hat{I}\pm$/Fe] vs. [Fe/H] space, with a roughly solar-$\hat{I}\pm$ sequence that spans a decade in metallicity and a high-$\hat{I}\pm$ sequence that merges with the low-$\hat{I}\pm$ sequence at super-solar [Fe/H]. The location of the high-$\hat{I}\pm$ sequence is nearly constant across the disk, however there are very few high-$\hat{I}\pm$ 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, with 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 $|z|>1$ kpc or [$\hat{I}\pm$/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.

Submitted, 18 pages, 16 figures