MASS MODELS OF THE MILKY-WAY

A parametrized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scalelength R-d,R- Of the stella r disc to R-0. The disc and bulge dominate upsilon(c)(R) at R less tha n or similar to R-0 only for R-d,R-/R-0 less than or similar to 0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radi i. When allowed this freedom, however, the halo causes changes in assu mptions relating to R much less than R-0 to affect profoundly the stru cture of the best-fitting model at R much greater than R-0. For exampl e, changing the disc slightly from an exponential surface-density prof ile significantly changes the form of upsilon(c)(R) at R much greater than R-0, where the disc makes a negligible contribution to upsilon(c) . Moreover, minor changes in the constraints can cause the halo to dev elop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct haloes rather than ou twards-increasing mass-to-light ratios. The mass distribution of the G alaxy and the relative importance of its various components will remai n very uncertain until more observational data can be used to constrai n mass models. Data that constrain the Galactic force field at z great er than or similar to R and at R > R-0 are especially important.