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Paper: The Galactic Halo Ionizing Field and H alpha Distances to HVCs
Volume: 166, Stromlo Workshop on High-Velocity Clouds
Page: 212
Authors: Bland-Hawthorn, Joss; Maloney, Philip R.
Abstract: There has been much debate in recent decades as to what fraction of ionizing photons from star forming regions in the Galactic disk escape into the halo. The recent detection of the Magellanic Stream in optical line emission at the CTIO 4m and the AAT 3.9m telescopes may now provide the strongest evidence that at least some of the radiation escapes the disk completely. While the distance to the Magellanic Stream is uncertain, the observed Ha emission is most plausibly explained by photoionization due to hot, young stars. Our model requires that the mean Lyman-limit opacity perpendicular to the disk is tauLL approx 3, assuming the covering fraction of the resolved clouds is close to unity. Within the context of this model, it now becomes possible to determine distances to high velocity clouds, and the 3D orientation of the Magellanic Stream. Here, we discuss complications of the model (eg porosity, topology), future tests, ongoing improvements, and the importance of Ha limb brightening from surface ionization. More speculatively, we propose a direct experiment for locating an HVC in 6-dimensional phase space above the Galactic plane.
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