Galactic Kinematics toward the South Galactic Pole: First Results from the Yale–San Juan Southern Proper Motion Program

Abstract

The predictions from a Galactic structure and kinematic model code are compared with the color counts and absolute proper motions derived from the Southern Proper Motion survey, covering more than 700 deg2 toward the south Galactic pole in the range 9 < BJ ≤ 19. The theoretical assumptions and associated computational procedures, the geometry for the kinematic model, and the adopted parameters are presented in detail and compared with other Galactic kinematic models of its kind. The data with which the model is compared consist of more than 30,000 randomly selected stars, and they are best fitted by models with a solar peculiar motion of +5 km s-1 in the V-component (pointing in the direction of Galactic rotation), a high LSR speed of 270 km s-1, and a (disk) velocity ellipsoid that always points toward the Galactic center. The absolute proper motions in the U-component indicate a solar peculiar motion of 11.0 ± 1.5 km s-1, with no need for a local expansion or contraction term. The fainter absolute motions show an indication that the thick disk must exhibit a rather steep velocity gradient of about -36 km s-1 kpc-1 with respect to the LSR. We are not able to set constraints on the overall rotation for the halo or on the thick disk or halo velocity dispersions. Some substructure in the U and V proper motions could be present in the brighter bins, 10 < BJ < 13, and it might be indicative of (disk) moving groups.