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 \sim 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 deg ^ { 2 } 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 \sigma _ { [ Fe / H ] } \sim 0.1 dex can be achieved for bulk stellar populations .
The global metal abundance of the Virgo Overdensity determined in this way is \langle [ Fe/H ] \rangle = -2.0 \pm 0.1 ( internal ) \pm 0.5 ( systematic ) , from photometric measurements of 0.7 million stars with heliocentric distances from \sim 10 kpc to \sim 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 .