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Paper: The Galactic Halo UV Field, Magellanic Stream, and High-Velocity Clouds
Volume: 240, Gas and Galaxy Evolution: A Conference in Honor of the 20th Anniversary of the VLA
Page: 369
Authors: Bland-Hawthorn, J.; Putman, M. E.
Abstract: Significant numbers of high-velocity HI clouds (HVCs) have now been detected in Hα, with a subset seen in low ionization lines (e.g., [N II]). It was originally hoped that the observed Hα strength would provide a distance constraint to individual clouds. This idea requires that a useful fraction (fesc > 1%) of ionizing photons escape the Galaxy, and that the halo ionizing field is relatively smooth, as we discuss. HVCs which are known to be close to the Sun are Hα-bright the brightest clouds also show enhanced [N II] emission, in contrast to the Magellanic Stream, where the low ionization emission lines are always weak compared to Hα. But an acute complication for Hα distances is the apparent Hα brightness of the Magellanic Stream along several sight lines, comparable to or brighter than local HVCs. To account for this, we present three possible configurations for the Magellanic Stream and propose a follow-up experiment. If we normalize the distances to local HVCs, some HVCs appear to be scattered throughout the Galactic halo on scales of tens of kiloparsecs.
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