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Composed primarily of hydrogen (91% by number) and helium (9%), with trace amounts (0.1%) of heavier elements, the interstellar medium plays a vital role in the cycle of stellar birth and death and galactic evolution. Not only do the properties of the interstellar medium govern the formation of new stars, but through their radiation and the matter and kinetic energy from their outflows and supernovae, these stars in turn determine the properties of the interstellar medium from which the next generation of stars will be born. One of these feedback processes, the subject of this review, is the large-scale ionization of the medium by the youngest and most luminous stars, the O stars. Even though they are located near the galactic midplane in rare, isolated regions of star formation and often surrounded by opaque clouds of neutral hydrogen, the Lyman continuum radiation from these hot stars is somehow able to propagate large distances through the disk and into the galaxy's halo to produce extensive ionization of the interstellar hydrogen. The study of this wide-spread plasma has impacted our understanding of the dynamic interstellar processes occurring in galaxies.

This area of study began more than four decades ago, when Hoyle and Ellis (1963) proposed to a skeptical astronomical community the existence of an extensive layer of warm (104 K), low density (10-1 cm-3) ionized hydrogen surrounding the plane of our Galaxy and having a power requirement comparable to the ionizing luminosity of the Galaxy's O and B stars. Their conclusion was based upon their discovery of a free-free absorption signature in the observations of the Galactic synchrotron background at frequencies between 1.5 and 10 MHz carried out by radio astronomy pioneers Grote Reber and G. R. A. Ellis at Hobart, Tasmania (Reber and Ellis 1956, Ellis et al. 1962). The idea that a significant fraction of the Galaxy's ionizing photons, produced primarily by rare, massive stars residing near the Galactic midplane, traveled hundreds of parsecs throughout the disk to produce wide-spread ionization conflicted with the traditional picture in which the neutral atomic hydrogen (the primary component of the medium by mass and opaque to hydrogen ionizing radiation) filled much of the interstellar volume. However, less than a decade later, the dispersion of radio signals from newly discovered pulsars (Hewish et al. 1968, Guélin 1974) plus the detection of faint optical emission lines from the diffuse interstellar medium (Reynolds 1971, Reynolds et al. 1973) firmly established warm ionized hydrogen as a major, wide-spread component of our Galaxy's interstellar medium. Two decades later, deep Halpha imaging with CCD detectors revealed that similar warm plasmas also permeate the disks and halos of other galaxies (Rand et al. 1990, Dettmar 1990).

The large mass, thickness, and power associated with this ionized layer has modified our understanding of the composition and structure of the interstellar medium and the distribution of Lyman continuum radiation within galaxies. Its weight is a major source of interstellar pressure at the midplane (Boulares and Cox 1990), and it could be the dominant state of the interstellar medium 1000 pc above the midplane (Reynolds 1991a). An accurate understanding of interstellar matter and processes thus requires as thorough a knowledge of this diffuse ionized gas as the other principal components of the medium. For a general survey of the interstellar medium, we refer the reader to The Interstellar Environment of Our Galaxy by Katia Ferrière (2001) and The Three Phase Interstellar Medium Revisited by Donald Cox (2005). Brief review articles on a variety of current interstellar medium topics can be found in How Does the Galaxy Work?, a conference proceedings edited by Alfaro et al. (2004).

Although the nature of this low density plasma is not yet fully understood, significant progress has been made in characterizing its properties and the source of its ionization. In Section 2, we present a brief account of observations of the warm ionized medium (WIM) in our Galaxy, the Milky Way, including results on the physical conditions in the WIM. In Section 3 and Section 4, we review progress on understanding this gas and its close connection to star formation activity and O stars through studies of other galaxies. O star photoionization models in a clumpy interstellar medium are discussed in Section 5, with a discussion of supplemental radiation from hot-cool gas interfaces in Section 6. A list of some unanswered questions and future challenges is presented in Section 7. Throughout, we keep the convention that WIM refers to the warm ionized medium in our Galaxy and DIG refers to this diffuse ionized gas in other galaxies. Also, we use H+ to refer to the ionized hydrogen in the low density, wide-spread WIM/DIG and H II to denote the gas in the localized, higher density, "classical H II regions" immediately surrounding hot stars. For reference, a list of acronyms and terms with explanations is provided at the end of this paper.

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