Dark Matter: Local Volume Density versus Total Surface Density

Abstract

Spectrophotometric data have been obtained with the 2dF spectrograph at Cerro Tololo Inter-American Observatory and with Hydra at the WIYN telescope for nearly 2000 A, F, and G stars toward the south Galactic pole. Using 1305 radial velocities, 2311 uvbyHβ photometric measurements, and 1621 Yale–San Juan SPM (Southern Proper Motion) absolute proper motions, peculiar velocities were derived to determine the galactic gravitational force K(z) perpendicular to the Galactic plane, first described by Oort (published in 1932). Our results in local volume density, as derived from early-type stars (A0–F5) and giants, support Bahcall's ρ = 0.1 M⊙ pc-3, implying that dark matter exists in the galactic disk. Our result for the total surface density derived using late-type stars (G1 and later) is equal to 34 M⊙ pc-2, about 30% smaller than 46 ± 9 M⊙ pc-2 computed by Kuijken & Gilmore, but greater than 23 M⊙ pc-2 listed for the extended halo mass for z < 1.1 kpc (Cox 1999). The derived behavior of K(z) versus distance from the Galactic plane shows a well-known peak at about 400 pc. A second peak at about 1100 pc exhibited by dwarf main-sequence stars, but not for the giants, suggests that this second peak is likely due to a thick-disk population of metal-poor objects.