The present and future mass of the Milky Way halo

A simple model for the Milky Way halo is presented. It has a flat rotation curve in the inner regions, but the density falls off sharply beyond an out er edge. This truncated, flat rotation curve (TF) model possesses a rich fa mily of simple distribution functions which vary in velocity anisotropy. Th e model is used to estimate the total mass of the Milky Way halo using the latest data on the motions of satellite galaxies and globular clusters at G alactocentric radii greater than 20 kpc. This comprises a data set of 27 ob jects with known distances and radial velocities, of which six also possess measured proper motions. Unlike earlier investigations, we find entirely c onsistent maximum likelihood solutions unaffected by the presence or absenc e of Leo I, provided both radial and proper motion data are used. The avail ability of the proper motion data for the satellites is crucial as, without them, the mass estimates with and without Leo I are inconsistent at the 99 per cent confidence level. All these results are derived from models in wh ich the velocity normalization of the halo potential is taken as similar to 220 km s(-1). A detailed analysis of the uncertainties in our estimate is presented, incl uding the effects of the small data set, possible incompleteness or correla tions in the satellite galaxy sample and the measurement errors. The most s erious uncertainties come from the size of the data set, which may cause a systematic underestimate by a factor of 2, and the measurement errors, whic h cause a scatter in the mass of the order of a factor of 2. We conclude th at the total mass of the halo is similar to 1.9(-1.7)(+3.6)x10(12) M-., whi le the mass within 50 kpc is similar to 5.4(-3.6)(+0.2)x10(11) M-.. In the near future, ground-based radial velocity surveys of samples of blue horizo ntal branch (BHB) stars are a valuable way to augment the sparse data set. A data set of similar to 200 radial velocities of BHB stars will reduce the uncertainty in the mass estimate to similar to 20 per cent. In the coming decade, microarcsecond astrometry will be possible with the Space Interfero metry Mission (SIM) and the Global Astrometry Interferometer for Astrophysi cs (GAIA) satellites. For example, GAIA can provide the proper motions of t he distant dwarfs like Leo I to within +/- 15 km s(-1) and the nearer dwarf s like Ursa Minor to within +/- 1 km s(-1). This will also allow the total mass of the Milky Way to be found to similar to 20 per cent. SIM and GAIA w ill also provide an accurate estimate of the velocity normalization of the halo potential at large radii.