The Barred Spiral Galaxy NGC 1398 and Its Pattern Speed

We have made observations of the 21 cm neutral hydrogen line of the barred spiral galaxy NGC 1398 at two configurations of the Very Large Array. The structure, dynamics and kinematics of the H I gas are studied over a spatial resolution range of 15" to 60", with a velocity resolution of 25.3 km s^-1. Surface density and radial velocity images are presented and the H I surface density is compared with B, V, and I band photometry. The neutral hydrogen surface density is complex, showing a large depleted region in the center of the galaxy. Beyond this, high-density segmented structure, coincident with the tightly wound optical spiral arms, is apparent in the high-resolution images. The central H I depletion is elongated perpendicular to the optical bar. No bar structure is observed in neutral hydrogen and there is only a weak association of H I gas with the optical ring. The gas disk extends well beyond the optical, to a diameter of more than 10'. The velocity field is symmetric and well ordered, with only small deviations from circular motion apparent at these resolutions. The rotation curve derived from the lowest spatial resolution data peaks and turns over at a radius of roughly one-half the total gas radius. We use this rotation curve to calculate the angular velocity and attempt to determine a pattern speed by associating prominent optical features with resonances. Using the inner optical ring and outer pseudoring(s) as markers of possible resonances, several models are explored, including one with two pattern velocities. None are found to be completely satisfactory.