Good Views of the Galaxy

Fitting Galactic structure models to star counts only provides useful information about the Galaxy in some directions. In this paper, we investigate the use of χ² goodness-of-fit tests to discriminate between degenerate Galactic structure models and the implications of this technique for the Galactic spheroid and thick-disk components. The axis ratio of the Galactic spheroid and the normalization of spheroid stars with respect to disk stars introduce a degenerate effect which means that Galactic structure models with certain combinations of these parameters are indistinguishable from each other in most directions. We present an analysis of the optimal directions in which these degeneracies can be lifted. Poisson and magnitude errors are taken into account, and an attempt is made to place an upper limit on the systematic error due to separation of spheroid stars from thick/old disk stars. We find that the magnitude range 20<V<21 is the best for lifting most degeneracies, and present the optimal combinations of directions using which this can be achieved. We also give directions in which the signature of the presence of a Galactic thick disk can be most readily identified, and the directions in which contamination from a thick disk can be minimized. It is hoped that forthcoming data from large-scale sky surveys would reveal much about the structure of our Galaxy using star count techniques.