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Published online by Cambridge University Press: 26 May 2016
There is now abundant evidence for high latitude gas in the Milky Way and external edge-on galaxies. The halo gas is seen in various ISM tracers, including HI, Hα, and radio continuum, and shows considerable structure, including discrete features which span the disk-halo interface. While some correlation with underlying star formation activity has been observed, several energy discrepancies are outlined which suggest that conventional processes related to massive star formation are not sufficient to account for the observed emission. Indeed, new sensitive data are showing that high latitude gas may be a common characteristic of all galaxies, not just those with high star formation rates.