HORIZONTAL BRANCH STARS IN THE METAL-WEAK THICK DISK AND INNER HALO

We present a complete spectroscopic survey of 2414 2MASS-selected blue horizontal branch (BHB) candidates selected over 4300 deg2 of the sky. We identify 655 BHB stars in this non-kinematically selected sample. We calculate the luminosity function of field BHB stars, and find evidence for very few hot BHB stars in the field. The BHB stars located at a distance from the Galactic plane |Z | < 4 kpc trace what is clearly a metal-weak thick disk population, with a mean metallicity of [Fe/H] = −1.7, a rotation velocity gradient of dvrot/d|Z | = −28 ± 3.4 km s−1 in the region |Z | < 6 kpc, and a density scale height of hZ = 1.26 ± 0.1 kpc. The BHB stars located at 5 < |Z | < 9 kpc are a predominantly inner-halo population, with a mean metallicity of [Fe/H] = −2.0 and a mean Galactic rotation of −4 ± 31 km s−1. We infer the density of halo and thick disk BHB stars is 104 ± 37 kpc−3 near the Sun, and the relative normalization of halo to thick-disk BHB stars is 4 ± 1 % near the Sun