2016ApJ...823...30B

The spatial structure of stellar populations with different chemical abundances in the Milky Way (MW) contains a wealth of information on Galactic evolution over cosmic time. We use data on 14,699 red-clump stars from the APOGEE survey, covering 4 kpc\lesssim R\lesssim 15 kpc , to determine the structure of mono-abundance populations (MAPs)-stars in narrow bins in [α/Fe] and [Fe/
H] -accounting for the complex effects of the APOGEE selection function and the spatially variable dust obscuration. We determine that all MAPs with enhanced [α/Fe] are centrally concentrated and are well-described as exponentials with a scale length of 2.2±0.2 kpc over the whole radial range of the disk. We discover that the surface-density profiles of low- [α/Fe] MAPs are complex: they do not monotonically decrease outwards, but rather display a peak radius ranging from ~5 to ~13 kpc at low [Fe/
H] . The extensive radial coverage of the data allows us to measure radial trends in the thickness of each MAP. While high- [α/Fe] MAPs have constant scale heights, low- [α/Fe] MAPs flare. We confirm, now with high-precision abundances, previous results that each MAP contains only a single vertical scale height and that low- [Fe/
H] , low- [α/Fe] and high- [Fe/
H] , high- [α/Fe] MAPs have intermediate ( {h}_{Z}~300–600 pc ) scale heights that smoothly bridge the traditional thin- and thick-disk divide. That the high- [α/Fe] , thick disk components do not flare is strong evidence against their thickness being caused by radial migration. The correspondence between the radial structure and chemical-enrichment age of stellar populations is clear confirmation of the inside-out growth of galactic disks. The details of these relations will constrain the variety of physical conditions under which stars form throughout the MW disk.