INTRODUCTION

With the advent of the AAO-UKST Galactic Plane H survey and the several other Galactic surveys discussed in this volume, an opportunity arises for progress in understanding the origin, structure, and ionization of DIG through the comparison of these surveys with observations of external edge-on galaxies. These galaxies offer the advantage of allowing the entire DIG layer to be quickly imaged (albeit at relatively low linear resolution), while avoiding the difficulty inherent in Milky Way observations of translating angular heights of emission to physical heights. Studies of the diffuse ionized layer of the Milky Way will allow, for example, the vertical structure of the layer and the dependence of line ratios on height from the Galactic plane and on the underlying disk environment to be compared with results from external edge-ons. The importance of DIG as an ISM component and the leverage it provides on understanding the ISM as a whole is discussed elsewhere in this volume by Reynolds, Ferrara, Elmegreen and others. The goal of this review is to summarize our understanding of DIG in edge-on galaxies at the time when these surveys are getting underway.

© Copyright Astronomical Society of Australia 1997