The galactic disk surface mass density and the Galactic force K(z) at Z = 1.1 kiloparsecs

Abstract

We rediscuss a set of distance and velocity data previously obtained and analyzed by ourselves to determine the surface mass density of the Galactic disk. We show that these data reliably determine the integral surface mass density of all (disk + halo) Galactic components within 1.1 kpc from the Galactic plane near the Sun to be 71 ± 6 M q pc-2 , independent of the disk/halo ratio. We show that determination of the fraction of this total mass which is distributed in the Galactic disk and the fraction which is associated with an extended halo remains highly model-dependent. Attempts to use extant data for stars far from the Galactic plane to determine the disk/halo mass fraction in the local surface density are likely to be unreliable. The best available estimate of the relative contributions of disk mass and halo mass to the local integral surface density, obtained from modeling of the Galactic rotation curve, yields a surface mass density of 48 ± 9 M 0 pc-2 for mass associated with the Galactic disk near the Sun. The corresponding mass of identified disk matter is 48 ± 8 M q pc-2. There remains no evidence for any significant unidentified mass in the Galactic disk.