The impact of ionizing stars on the diffuse interstellar medium: A swept-up shell and ionized halo around the W4 chimney/superbubble

Normandeau et al. have suggested that W4 is a Galactic chimney powered by the OCl 352 cluster, and subsequent work adds to its status as the prime candidate chimney in our Galaxy. The model proposes that the massive star cluster OCl 352 powers a superbubble that achieved "blowout'' to form a Galactic chimney. In this picture blowout occurs above the Galactic plane because of a density gradient in the interstellar medium. Using the similar to1' resolution far-infrared IRAS IGA and radio DRAO CGPS Galactic plane surveys, we analyze the neutral and ionized components of the W4 chimney/superbubble. We show that W4 has a swept-up, partially ionized shell of gas and dust that is powered by the OCl 352 star cluster. Dust optical depth maps show dense interstellar material below the shell and provide direct evidence of the dense gas that caused the lower shell expansion to stall. Although parts of the shell are found to be ionization-bounded, the shell is very inhomogenous, and an ionized halo provides evidence of significant Lyman continuum leakage. A large fraction (40%) of the OCl 352 cluster photons escape to large distances and are available to ionize the Galactic halo and the diffuse ionized gas component of the interstellar medium.