Photometric metallicity calibration with SDSS and SCUSS and its application to distant stars in the south Galactic cap

Based on Sloan Digital Sky Survey (SDSS) g, r and South Galactic Cap of u-band Sky Survey (SCUSS) u photometry, we develop a photometric calibration for estimating the stellar metallicity from u - g and g - r colours by using the SDSS spectra of 32 542 F- and G-type main-sequence stars, which cover almost 3700 deg² in the south Galactic cap. The rms scatter of the photometric metallicity residuals relative to spectrum-based metallicity is 0.14 dex when g - r < 0.4, and 0.16 dex when g - r > 0.4. Because of the deeper and more accurate magnitude of SCUSS u band, the estimate can be used up to the faint magnitude of g = 21. This application range of photometric metallicity calibration is wide enough so that it can be used to study metallicity distribution of distant stars. In this study, we select the Sagittarius (Sgr) stream and its neighbouring field halo stars in south Galactic cap to study their metallicity distribution. We find that the Sgr stream at the cylindrical Galactocentric coordinate of R ~ 19 kpc, |z| ~ 14 kpc exhibits a relative rich metallicity distribution, and the neighbouring field halo stars in our studied fields can be modelled by two-Gaussian model, with peaks, respectively, at [Fe/H]=-1.9 and -1.5.