Solar-neighbourhood observations and the structure of the Galaxy

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Abstract
Solar-neighbourhood kinematics are linked to the large-scale structure of the Galaxy by Newton)s laws of motion. Simple examples are used to explore the nature of this connection, and to develop effective strategies for fitting dynamical models of the Galaxy to the kind of data that will become available in the next decade. About a third of the spheroid)s mass could be tracked through observations confined to stars near the Sun. A set of deep radial-velocity surveys well distributed over one galactic hemisphere would contain more than enough information to determine the orbits of the remaining two thirds of the spheroid stars. Complexities in the way information about the distribution of stars in the Galaxy)s phase space is folded into observational data, suggest that direct inversion of the data to yield the spheroid)s distribution function, though in principle possible, is likely to be a less useful procedure than model fitting. Several examples of fitting models to pseudo-data illustrate the ability of local and line-of-sight velocity surveys to discriminate between dynamical models that satisfy the obvious constraints on stellar densities and velocity dispersions. Three appendices discuss resonant orbits in the solar neighbourhood, Eggen)s groups of high-velocity stars, and a generalization of the Lindblad diagram.

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