A SEARCH FOR DISTANT STARS IN THE MILKY-WAY GALAXY HALO AND THICK DISK

We have observed 247 faint (13.3 mag < V < 17.9 mag) stars with B - V less-than-or-equal-to 1.00 in one square degree near the North Galactic Pole. Most of these stars are distant, lying in either the Galactic halo or thick disk, and allow a study of these two populations in situ, with no kinematic or metallicity bias. For all 247 stars, spectra yield w velocities, and for over half the stars, Stromgren photometry gives luminosities, distances, and metallicities. Even though it was selected with no kinematic or metallicity criteria, the sample consists predominantly of thick disk and halo stars: the velocity dispersion of the total sample is 62 km s-1. The farthest star has a Galactocentric distance of 30 kpc; the fastest has w = - 295 km s-1. For the stars with known distances and metallicities, three populations appear: a thin disk at solar metallicities, a thick disk ranging in metallicity from about [Fe/H] = - 0.5 to [Fe/H] = - 1 with a velocity dispersion sigma-w of 30-35 km s-1, and a halo with metallicity [Fe/H] < - 1 and a velocity dispersion sigma-w of 60 km s-1. However, stars with unknown distances and metallicities-which are the faintest and presumably farthest stars in the survey-have sigma-w = 80 km s-1, suggesting that the more distant halo has a velocity dispersion considerably in excess of 60 km s-1. We find no abundance gradient in the outer (R > R0) halo. Stars we can reliably assign to the thick disk are red and old, consistent with their being similar in age to metal-rich globulars. If bound to the Galaxy, one faint distant high-velocity giant star implies that the mass of the Galaxy out to 30 kpc must be at least 1.4 x 10(11) M.. Another faint star, blue in color but of unknown luminosity and distance, implies that the Galactic mass out to 30 kpc must be at least 3 x 10(11) M., if the star is on the horizontal branch. We find no evidence for an imbalance in the motions of the halo stars; nor for any O, B, or A stars; nor for any zero-metal stars. However, about 10% of the "stars" turned out to be galaxies. Finally, a possible halo moving group, with [Fe/H] almost-equal-to - 1.7 and w almost-equal-to - 30 km s-1, appears in the sample, but its existence is doubtful.