THE MASS OF THE MILKY-WAY GALAXY

We present a method for determining the Galactic mass and mass distribution based on maximum likelihood parameter estimation. We show that the number density of the Galaxy's outer satellites can be modelled by a power-law function. Assuming that the total distribution of matter (visible and dark) can be modelled in a similar way, we construct a self-consistent, self-similar model of the outer Galaxy. From this we derive a probability distribution for two quantities; the radial velocity and galactocentric distance. Considering the velocity distribution in the outer Galaxy to be isotropic, this probability distribution contains two free parameters; the Galactic mass and the power-law index for the total matter density profile which we subsequently determine by way of maximum likelihood estimation. We obtain a mass of around 1.3 x 10(12) M. out to a distance of 230 kpc from the Galactic Centre with a corresponding power-law index of 2.4. Confidence regions are derived for these estimates from which a 'low-mass' galaxy, (i.e. M < 6 x 10(11) M.), can be ruled out with 98 per cent confidence. Our result suggests a high mass for the Milky Way out to a large distance and relies on the inclusion of the dwarf spheroidal galaxy Leo I in the sample. The most recent velocity and distance data we could find for all globular clusters and dwarf spheroidal galaxies, beyond R0, including the newly discovered dwarf spheroidal galaxy in Sextans were used in the analysis. We also briefly discuss a self-similar galactic model where the velocity distribution is anisotropic.