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 stellar disc to R-0. The disc and bulge dominate upsilon(c)(R) at R less than 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 radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R much less than R-0 to affect profoundly the structure of the best-fitting model at R much greater than R-0. For example, changing the disc slightly from an exponential surface-density profile 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 develop 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 outwards-increasing mass-to-light ratios. The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z greater than or similar to R and at R > R-0 are especially important.