VLA SYNTHESIS OF H-I ABSORPTION TOWARD W43 (G30.8+0.0)

We used the VLA to synthesize the continuum structure of the main comp onent of W43, a very large H II region located near the subcentral poi nt at l = 30.8-degrees, b = 0.00-degrees, and we mapped the 21 cm H I absorption across it at resolution 12.5'' and 1.29 km s-1. The H I occ urs in features which strikingly show the influence of spiral structur e on low-latitude galactic kinematics. The observations can be underst ood in terms of the two-arm spiral-shock model of Roberts & Burton [To pics in Interstellar Matter, edited by H. van Woerden (Reidel, Dordrec ht), p. 195 (1977)], with the additional requirement that the mean H I spin temperature be a strong function of spiral phase. This ansatz al so has the virtue of explaining a long-standing anomaly in the overall shape of low-latitude inner-galaxy H I emission profiles, which often fail to show the strong terminal velocity ridge imposed by simple rot ational kinematics in a uniform gas.