AAS 196th Meeting, June 2000
Session 26. The Galactic ISM: Observations and Modelling
Display, Tuesday, June 6, 2000, 10:00am-6:30pm, Empire Hall South

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[26.09] The Contribution of Ionizing Stars to the FIR and Radio Emission in the Milky Way: Evidence for a Swept-up Shell and Diffuse Ionized Halo around the W4 Chimney/Superbubble

S. Terebey (Extrasolar Research Corp), M. Fich (Univ of Waterloo), R. Taylor (Univ of Calgary), Y. Cao (KLA Tencor), T. Hancock (Extrasolar Research Corp)

Normandeau et al (1996) have suggested that W4 is a galactic chimney on the basis of an HI void, the only chimney to-date identified in our Galaxy. The model proposes that the massive star cluster OCl~352 powers a superbubble which achieved ``breakout" to form a Galactic chimney. Breakout occurs above the Galactic plane due to a density gradient in the interstellar medium. Using the recent ~1' 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 W4 has a swept-up partially ionized shell of gas and dust which 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 which 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 percent) of the OCl~352 cluster photons escape to large distances and are available to ionize the Galactic halo and the diffuse ionized gas (DIG) component of the interstellar medium.

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