The Century Survey Galactic Halo Project. II. Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars

We discuss a 175 deg^2 spectroscopic survey for blue horizontal branch (BHB)stars in the Galactic halo. We use the Two Micron All Sky Survey (2MASS) andthe Sloan Digital Sky Survey (SDSS) to select BHB candidates, and find that the2MASS and SDSS color-selection is 38% and 50% efficient, respectively, for BHBstars. Our samples include one likely run-away B7 star 6 kpc below the Galacticplane. The global properties of the BHB samples are consistent with membershipin the halo population: the median metallicity is [Fe/H]=-1.7, the velocitydispersion is 108 km/s, and the mean Galactic rotation of the BHB stars3<|z|<15 kpc is -4 +- 30 km/s. We discuss the theoretical basis of the Preston,Shectman & Beers M_V-color relation for BHB stars, and conclude that intrinsicshape of the BHB M_V-color relation results from the physics of stars on thehorizontal branch. We calculate the luminosity function for the field BHB starsamples using the Efstathiou, Ellis, & Peterson maximum-likelihood method whichis unbiased by density variations. The field BHB luminosity function exhibits asteep rise at bright luminosities, a peak between 0.8 < M_V < 1.0, and a tailat faint luminosities. We compare the field BHB luminosity functions with theluminosity functions derived from sixteen different globular cluster BHBs.Kolmogorov-Smirnov tests suggest that field BHB stars and BHB stars in globularclusters share a common distribution of luminosities, with the exception ofglobular clusters with extended BHBs.Comment: 14 pages, including 16 figures, accepted for publication in A