The Century Survey Galactic Halo Project III: A Complete 4300 deg^2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo

TL;DR

This survey provides a comprehensive spectroscopic analysis of blue horizontal branch stars across 4300 square degrees, revealing their distribution, metallicity, and kinematic properties in the Galactic halo and thick disk.

Contribution

It offers the first complete spectroscopic survey of BHB stars over a large sky area, distinguishing between thick disk and halo populations based on metallicity and kinematics.

Findings

Thick disk BHB stars have a mean metallicity of [Fe/H]= -1.7.

Halo BHB stars have a mean metallicity of [Fe/H]= -2.0.

Halo BHB stars constitute about 4% of the local BHB population.

Abstract

We present a complete spectroscopic survey of 2414 2MASS-selected blue horizontal branch (BHB) candidates selected over 4300 deg^2 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 dv_{rot}/d|Z|= -28+-3.4 km/s in the region |Z|<6 kpc, and a density scale height of h_Z= 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. 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.