The Vertical Metallicity Gradient of the Milky Way Disk: Transitions in
  [a/Fe] Populations
  Using G dwarfs from the Sloan Extension for Galactic Understanding and
Exploration (SEGUE) survey, we have determined a vertical metallicity gradient
over a large volume of the Milky Way's disk, and examined how this gradient
varies for different [a/Fe] subsamples. This sample contains over 40,000 stars
with low-resolution spectroscopy over 144 lines of sight. We employ the SEGUE
Stellar Parameter Pipeline (SSPP) to obtain estimates of effective temperature,
surface gravity, [Fe/H], and [a/Fe] for each star and extract multiple
volume-complete subsamples of approximately 1000 stars each. Based on the
survey's consistent target-selection algorithm, we adjust each subsample to
determine an unbiased picture of the disk in [Fe/H] and [a/Fe]; consequently,
each individual star represents the properties of many. The SEGUE sample allows
us to constrain the vertical metallicity gradient for a large number of stars
over a significant volume of the disk, between ~0.3 and 1.6 kpc from the
Galactic plane, and examine the in situ structure, in contrast to previous
analyses which are more limited in scope. This work does not pre-suppose a disk
structure, whether composed of a single complex population or a distinct thin
and thick disk component. The metallicity gradient is -0.243 +0.039 -0.053
dex/kpc for the sample as a whole, which we compare to various literature
results. Each [a/Fe] subsample dominates at a different range of heights above
the plane of the Galaxy, which is exhibited in the gradient found in the sample
as a whole. Stars over a limited range in [a/Fe] show little change in median
[Fe/H] with height. If we associate [a/Fe] with age, our consistent vertical
metallicity gradients with [a/Fe] suggest that stars formed in different epochs
exhibit comparable vertical structure, implying similar star-formation
processes and evolution.
