ON THE DYNAMICAL EVOLUTION OF H ii REGIONS: AN INVESTIGATION OF THE IONIZED COMPONENT OF W4, A GALACTIC CHIMNEY CANDIDATE. I. KINEMATICS AND DYNAMICS IN THE LATITUDE RANGE 0 ◦ b 3◦

Fabry-Perot interferometry was used to obtain an Hα survey of the most emissive part of W4, a giant superbubble/ H ii region located in the Perseus arm. Presented by Normandeau and colleagues as an H i cavity aiming away from the Galactic plane, the void has been morphologically interpreted as a Galactic chimney candidate in interaction with the Galactic corona. We present the kinematical results of nearly five million Hα spectra obtained in the southern portion of the nebula (0 ◦ b 3◦). Many small-scale radial velocity gradients are detected in the embedded ionized component and are attributed to the photoionization of dense, mostly molecular, fragments found either in or at the periphery of the expanding supershell. The mean local standard of rest radial velocity associated with our Hα survey is found at −42.565 ± 5.204 (1σ) km s−1, redshifted by roughly 5 km s−1 from the molecular material found in the vicinity of the large superbubble. Investigation of the Hα line-width measurements has shown W4-south to fall in a transient regime between low velocity dispersions characteristic of small-size Galactic H ii regions and supersonic line widths associated with supergiant extragalactic structures. The overall kinematics of W4-south is best explained with the Champagne model for the dynamical evolution of H ii regions where at least 10 independent gas flows crisscross the nebula. For the first time, a Champagne flow is seen coming to an end within a nebula, mingling with the surrounding ionized gas. The nature (molecular versus atomic) of the neutra