The local surface density of disc matter mapped by Hipparcos

Abstract

We determine the surface density of matter in the disc of the Galaxy at the solar position using K giant stars. The local space density and luminosity function of the giants is determined using parallaxes from the Hipparcos satellite; for more distant giants, observed in a cone at the South Galactic Pole, distances are determined using intermediate-band DDO photometry (which has been calibrated to the Hipparcos absolute magnitudes). From this sample, we determine the gravitational potential vertically of the local Galactic disc, by comparing the number of giant stars observed in the cone with the number expected for various models of the matter distribution in the disc. We derive an estimate of the dynamical disc mass surface density of 56 ± 6 M ⊙ pc-2, which may be compared to an estimate of 53 M⊙ pc-2 in visible matter. For all gravitating matter (disc + dark halo) we find the total density within 1.1 kpc of the disc midplane to be 74 ± 6 M⊙ pc-2. As has been found by a number of studies, including our own, we find no compelling evidence for significant amounts of dark matter in the disc.