DIFFUSE IONIZED-GAS IN EDGE-ON GALAXIES

We review observations of diffuse ionised gas (DIG) in edge-on spiral galaxies. Deep imaging has revealed a variety of morphologies for 'ext raplanar' (above the HII region layer) or 'halo' DIG: some show widesp read prominent layers of truly diffuse gas and filamentary structures: others show just one patch 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 la yer is the star-formation activity. Smaller-scale connections between DIG and local star formation activity can he seen in some of the galax ies. DIC halos ase also related to radio continuum,;ri-ray and HI halo s. Those galaxies with the most prominent DIG layers also have the mos t prominent radio halos. In NGC 891, there are spatial correlations be tween extraplanar DIG, radio emission, X-ray emission and HI, and thes e in turn are related to the underlying disk star-formation activity. Progress on identifying the sources of ionisation and heating of the g as has come from spectroscopy, imaging in different emission lines; an d Fabry-Perot observations. By comparing with theoretical predictions, much evidence is found for photoionisation by disk stars as the prima ry ionisation and heating mechanism: but recent observations are begin ning to point towards the need for additional sources of ionisation an d/or heating.