The Metallicity Distribution of the Milky Way Bulge
  The Galactic bulge of the Milky Way is made up of stars with a broad range of
metallicity, -3.0 < [Fe/H] < 1 dex. The mean of the Metallicity Distribution
Function (MDF) decreases as a function of height z from the plane and, more
weakly, with galactic radius. The most metal rich stars in the inner Galaxy are
concentrated to the plane and the more metal poor stars are found predominantly
further from the plane, with an overall vertical gradient in the mean of the
MDF of about -0.45 dex/kpc. This vertical gradient is believed to reflect the
changing contribution with height of different populations in the inner-most
region of the Galaxy. The more metal rich stars of the bulge are part of the
boxy/peanut structure and comprise stars in orbits which trace out the
underlying X-shape. There is still a lack of consensus on the origin of the
metal poor stars ([Fe/H] < -0.5) in the region of the bulge. Some studies
attribute the more metal poor stars of the bulge to the thick disk and stellar
halo that are present in the inner region, and other studies propose that the
metal poor stars are a distinct "old spheroid" bulge population. Understanding
the origin of the populations that make up the MDF of the bulge, and
identifying if there is a unique bulge population which has formed separately
from the disk and halo, has important consequences for identifying the relevant
processes in the the formation and evolution of the Milky Way.
