Introduction

Warm Ionized Medium (WIM), or Diffuse Ionized Gas, is the dominant component of the ionized Interstellar Medium (ISM) in disk galaxies. While the H emission from this component is 10 to 1000 times fainter than for traditional HII regions, and the gas has low density ( cm), its large volume filling factor and spatial extent imply that the mass of the WIM easily surpasses that contained in traditional HII regions or in the hot gas in the ISM. Understanding the heating and ionization mechanism for the WIM is a major challenge to models of the ISM. In external galaxies we can determine the overall distribution and morphology of the WIM across galactic disks, its correlation with other ISM phases, and the variation in its properties with Hubble type and star formation rate. In addition, we can test ionization models for the WIM through spectroscopy, and through determining the relation between WIM and ionizing stars. In this paper we will review results for galaxies that are not edge-on; see Rand (this volume) for results on edge-on systems.

© Copyright Astronomical Society of Australia 1997