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Polar ring systems are elliptical or S0 galaxies, with a ring of dust, gas and stars orbiting nearly perpendicular to the projected major axis of the central galaxy. The kinematics of the polar material offers one of the few opportunities to probe the three-dimensional structure of galactic dark halos. We have used the VLA in C-array to obtain high-resolution HI velocity fields for four of the closest systems with nearly edge-on rings. Our observations have a spatial resolution of 15" and a velocity resolution of 10.5km/s. Total HI masses for all four objects (NGC~5122, A~0136-0801, UGC~7576, and II~Zw~73) are at least a billion solar masses, and the gas in each case is associated with the polar ring, not with the central galaxy. In NGC 5122 the gas extends well beyond the optical extent of the ring, while in the other systems the HI appears coextensive with the stellar ring. We detected nearby companions for NGC~5122 and II~Zw~73, but the polar rings do not appear to have been disturbed by the presence of these companions. Three of the polar rings (A~0136-0801, UGC~7576, and II~Zw~73) show slight warps in the outer HI distribution.
The observations of NGC~5122 presented here have sufficient resolution for detailed kinematic modeling; our results indicate that the kinematic minor and major axes are skewed away from those of the HI distribution. This is a signature of oval orbits, which indicates that the galaxy potential (and so possibly the dark halo) is somewhat flattened. Detailed modelling is in progress. We anticipate higher-resolution B-array observations of UGC~7576 and II~Zw~73, which will give a similar physical resolution for these galaxies.
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