The Milky Way Tomography with SDSS. II. Stellar Metallicity

Abstract

Using effective temperature and metallicity derived from SDSS spectra for ~60,000 F- and G-type main-sequence stars (0.2 < g − r < 0.6), we develop polynomial models for estimating these parameters from the SDSS u − g and g − r colors. These photometric estimates have similar error properties as those determined from SDSS spectra. We apply this method to SDSS photometric data for over 2 million F/G stars and measure the unbiased metallicity distribution for a complete volume-limited sample of stars at distances between 500 pc and 8 kpc. The metallicity distribution can be exquisitely modeled using two components with a spatially varying number ratio, which correspond to disk and halo. The two components also possess the kinematics expected for disk and halo stars. The metallicity of the halo component is spatially invariant, while the median disk metallicity smoothly decreases with distance from the Galactic plane from –0.6 at 500 pc to –0.8 beyond several kiloparsecs. The absence of a correlation between metallicity and kinematics for disk stars is in a conflict with the traditional decomposition in terms of thin and thick disks. We detect coherent substructures in the kinematics-metallicity space, such as the Monoceros stream, which rotates faster than the LSR, and has a median metallicity of [Fe/H] = −0.95, with an rms scatter of only ~0.15 dex. We extrapolate our results to the performance expected from the Large Synoptic Survey Telescope (LSST) and estimate that LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper-motion measurements accurate to ~0.5 mas yr−1, for about 200 million F/G dwarf stars within a distance limit of ~100 kpc (g < 23.5).