  The spatial, kinematic, and elemental-abundance structure of the Milky Way's
stellar disk is complex, and has been difficult to dissect with local
spectroscopic or global photometric data. Here, we develop and apply a rigorous
density modeling approach for Galactic spectroscopic surveys that enables
investigation of the global spatial structure of stellar sub-populations in
narrow bins of [\alpha/Fe] and [Fe/H], using 23,767 G-type dwarfs from
SDSS/SEGUE. We fit models for the number density of each such mono-abundance
component, properly accounting for the complex spectroscopic SEGUE sampling of
the underlying stellar population. We find that each mono-abundance
sub-population has a simple spatial structure that can be described by a single
exponential in both the vertical and radial direction, with continuously
increasing scale heights (~200 pc to 1 kpc) and decreasing scale lengths (>4.5
kpc to 2 kpc) for increasingly older sub-populations, as indicated by their
lower metallicities and [\alpha/Fe] enhancements. That the abundance-selected
sub-components with the largest scale heights have the shortest scale lengths
is in sharp contrast with purely geometric `thick--thin disk' decompositions.
To the extent that [\alpha/Fe] is an adequate proxy for age, our results
directly show that older disk sub-populations are more centrally concentrated,
which implies inside-out formation of galactic disks. The fact that the largest
scale-height sub-components are most centrally concentrated in the Milky Way is
an almost inevitable consequence of explaining the vertical structure of the
disk through internal evolution. Whether the simple spatial structure of the
mono-abundance sub-components, and the striking correlations between age, scale
length, and scale height can be plausibly explained by satellite accretion or
other external heating remains to be seen.
