Metallicity Mapping with gri Photometry: The Virgo Overdensity and the Halos of the Galaxy

TL;DR

This paper presents a method to estimate stellar metallicity using SDSS gri photometry, enabling large-scale mapping of metallicity in the Galaxy's halo, exemplified by the Virgo Overdensity.

Contribution

The study introduces a new photometric metallicity estimation technique using gri bands, extending metallicity mapping beyond 10 kpc and providing insights into halo composition.

Findings

Photometric metallicity estimates achieve ~0.1 dex precision for populations.

The Virgo Overdensity has a mean metallicity of approximately -2.0 dex.

Metallicity maps show a gradient from inner to outer halo regions.

Abstract

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.