The Milky Way Tomography with SDSS: II. Stellar Metallicity

TL;DR

This study develops models to estimate stellar metallicity from SDSS colors, revealing the spatial distribution and kinematics of disk and halo stars, and identifies substructures like the Monoceros stream, with implications for future LSST observations.

Contribution

It introduces polynomial models for metallicity estimation from SDSS colors and characterizes the spatial, kinematic, and metallicity properties of Galactic components and substructures.

Findings

Metallicity distribution modeled with two components: disk and halo.

Halo metallicity is spatially invariant, disk metallicity decreases with height.

Detection of substructures like the Monoceros stream with distinct kinematics.

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. We apply this method to SDSS photometric data for about 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, that 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 kpc. 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]=-0.96, 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 the LSST will obtain metallicity measurements accurate to 0.2 dex or better, with proper motion measurements accurate to ~0.2 mas/yr, for about 200 million F/G dwarf stars within a distance limit of ~100 kpc (g<23.5). [abridged]