The mass distribution in the galactic disc - II. Determination of the surface mass density of the galactic disc near the Sun

Abstract

The combination of a spatial density distribution and velocity–distance data for a stellar tracer population allows the determination of both the gravitational force law perpendicular to the galactic plane, and the total surface mass density of the galactic disc. We have obtained new photometric and spectroscopic data for a sample of K dwarfs towards the south galactic pole, and analysed them using the distribution function modelling technique described in the accompanying Paper I (Kuijken & Gilmore). The surface mass density of identified mass can also be found, by integrating the stellar population near the Sun through the derived K_z force law, and adding to that the observed amount of interstellar gas. The total surface mass density of the galactic disc derived from the dynamical analysis is $$46\pm9\enspace M_\odot\enspace \text{pc}^{-2}$$. The total identified surface mass density of the galactic disc near the Sun is $$48\pm8\enspace M_\odot\enspace \text{pc}^{-2}$$. Thus, there is no dynamically significant missing mass in the galactic disc near the Sun, though there is certainly a massive dark halo. Some implications of this result are discussed.