2011AJ....142..184C

Based on SDSS DR8, we have selected a sample of 1728 red horizontal-branch (RHB) stars with|Z| < 3 kpc by using a color-metallicity relation and stellar parameters. The sample stars clearly trace a typical thick disk population with peaks at|Z| = 1.26 kpc and [Fe/H] = -0.54. The vertical metallicity gradient of the thick disk is estimated in two ways. One is a fit to the Gaussian peaks of the metallicity histograms of the thick disk by subtracting minor contributions from the thin disk and the inner halo based on the Besanon Galaxy Model. The resulting gradient is -0.12±0.01 dex/kpc for 0.5 <|Z| < 3 kpc. The other method is to linearly fit the data based on stars with 1 <|Z| < 3 kpc being the main component of the thick disk. Five subgroups are then selected in different directions in the X-|Z| plane to investigate the difference in the vertical metallicity gradient between the Galactocenter and anti-Galactocenter directions. We found that a vertical gradient of -0.22 ±0.07 dex/kpc is detected for five directions except for one involving the pollution of stars from the bulge. The results indicate that the vertical gradient is dominant, but a radial gradient has a minor contribution for the thick disk population represented by RHB stars with 1 <|Z| < 3 kpc. The present work strongly suggests the existence of a metallicity gradient in the thick disk, which is thought to be negligible in most previous works in the literature.