The Stellar Metallicity Distribution Function of the Galactic Halo from
  SDSS Photometry
  We explore the stellar metallicity distribution function of the Galactic halo
based on SDSS ugriz photometry. A set of stellar isochrones is calibrated using
observations of several star clusters and validated by comparisons with
medium-resolution spectroscopic values over a wide range of metal abundance. We
estimate distances and metallicities for individual main-sequence stars in the
multiply scanned SDSS Stripe 82, at heliocentric distances in the range 5 - 8
kpc and |b| > 35 deg, and find that the in situ photometric metallicity
distribution has a shape that matches that of the kinematically-selected local
halo stars from Ryan & Norris. We also examine independent kinematic
information from proper-motion measurements for high Galactic latitude stars in
our sample. We find that stars with retrograde rotation in the rest frame of
the Galaxy are generally more metal poor than those exhibiting prograde
rotation, which is consistent with earlier arguments by Carollo et al. that the
halo system comprises at least two spatially overlapping components with
differing metallicity, kinematics, and spatial distributions. The observed
photometric metallicity distribution and that of Ryan & Norris can be described
by a simple chemical evolution model by Hartwick (or by a single Gaussian
distribution); however, the suggestive metallicity-kinematic correlation
contradicts the basic assumption in this model that the Milky Way halo consists
primarily of a single stellar population. When the observed metallicity
distribution is deconvolved using two Gaussian components with peaks at [Fe/H]
~ -1.7 and -2.3, the metal-poor component accounts for ~20% - 35% of the entire
halo population in this distance range.
