Diffuse ionized gas in edge-on galaxies

We review observations of diffuse ionized gas (DIG) in edge-on spiral galaxies. Deep imaging has revealed a variety of morphologies for `extraplanar' (above the HII region layer) or `halo' DIG: some show widespread prominent layers of truly diffuse gas and filamentary structures, others show just one or a few patches of extraplanar emission, while still others show no detectable extraplanar DIG at all. The key galactic property which appears to govern the prominence of the DIG layer is the star formation activity. Smaller scale connections between DIG and local star formation activity can be seen in some of the galaxies. DIG halos are also related to radio continuum, X-ray and HI halos. Those galaxies with the most prominent DIG layers also have the most prominent radio halos. In NGC 891, there are spatial correlations between extraplanar DIG, radio emission, X-ray emission and HI, and these in turn are related to the underlying disk star formation activity. Progress on identifying the sources of ionization and heating of the gas has come from spectroscopy, imaging in different emission lines, and Fabry-Perot observations. By comparing with theoretical predictions, much evidence is found for photo-ionization by disk stars as the primary ionization and heating mechanism, but recent observations are beginning to point towards the need for additional sources of ionization and/or heating.