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2. The Shape of the Dark Halo

The existence of stars and gas in two nearly perpendicular planes provides the opportunity of mapping the shape of the gravitational potential in three dimensions. Based on the observations of three polar-ring systems, Whitmore, McElroy and Schweizer (1987a, hereafter WMS) find that the massive dark halos around these galaxies are nearly spherical. However, a more detailed set of mass models have been constructed for NGC 4650A by Sackett and Sparke (1990a, b), who find that a more flattened dark halo is also compatible with the data for this particular galaxy.