Metallicity Mapping with gri Photometry: The Virgo Overdensity and the
  Halos of the Galaxy
  We describe the methodology required for estimation of photometric estimates
of metallicity based on the SDSS gri passbands, which can be used to probe the
properties of main-sequence stars beyond ~ 10 kpc, complementing studies of
nearby stars from more metallicity-sensitive color indices that involve the u
passband. As a first application of this approach, we determine photometric
metal abundance estimates for individual main-sequence stars in the Virgo
Overdensity, which covers almost 1000 square degrees on the sky, based on a
calibration of the metallicity sensitivity of stellar isochrones in the gri
filter passbands using field stars with well-determined spectroscopic metal
abundances. Despite the low precision of the method for individual stars,
internal errors of in [Fe/H] ~ +/- 0.1 dex can be achieved for bulk stellar
populations. The global metal abundance of the Virgo Overdensity determined in
this way is <[Fe/H]> = -2.0 +/- 0.1 (internal) +/- 0.5 (systematic), from
photometric measurements of 0.7 million stars with heliocentric distances from
~ 10 kpc to ~ 20 kpc. A preliminary metallicity map, based on results for 2.9
million stars in the northern SDSS DR-7 footprint, exhibits a shift to lower
metallicities as one proceeds from the inner- to the outer-halo population,
consistent with recent interpretation of the kinematics of local samples of
stars with spectroscopically available metallicity estimates and full space
motions.
