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As with many topics dependent on optical astronomy, the introduction of CCDs into mainstream research in the early 1990s propelled the study of the WIM to a new level. The increased sensitivity and linearity allowed new investigations of the morphology and extent of faintly emitting gas in the Milky Way (Section 4), providing a new view of the relationship between H ii regions and the diffuse gas. The WIM in other spiral galaxies could also be studied to similar physical extents from the plane as our galaxy, showing that this component of our ISM was ubiquitous in similar galaxies.

Deep imaging and spectroscopy of the diffuse gas of many spiral and irregular star-forming galaxies provided a welcome global view in the early 1990s. Since the fainter emitting gas in other galaxies was not necessarily known a priori to be similar to the WIM of the Milky Way, extragalactic researchers called it "diffuse ionized gas" (DIG). Today it is generally recognized that most of the DIG emission traces the WIM component in a galaxy. Halpha imaging of edge-on spirals reveals a large, extended, diffuse ionized component that generally matches the inferred distribution of the Milky Way from our interior vantage point (Figure 1). Complementary observations of face-on galaxies show diffuse emission following star formation and the general structure of the spiral arms. Spectroscopy of the diffuse gas in a variety of galaxy types and orientations replicate general trends in the Milky Way but exhibit variation in metallicity and excitation present in such a larger observational sample.

Figure 1

Figure 1. The WIM in NGC 891. A continuum-subtracted Halpha image of this nearly edge-on galaxy. Adapted from Howk & Savage (2000).

The deluge of observations from these initial studies provided a global view that further reinforced the WIM as a distinct component of the ISM in spirals:

Rossa & Dettmar (2003a, 2003b) completed one of the most comprehensive Halpha imaging surveys of spirals to date. Their study shows a clear link between infrared tracers of active star formation and the existence of an extended ionized layer in a galaxy. Such a relationship suggests that either the source responsible for powering the star formation or a byproduct of the formation itself is responsible for maintaining the WIM layer. Although this link is consistent with ionization by massive stars, it does not rule out a role played by other sources such as supernovae.

Evidence for a closer link between the massive stars and the WIM has come from the work of Ferguson et al. (1996) and Zurita, Rozas, & Beckman (2000) . In these studies of the ionized gas in moderately inclined to face-on spirals they trace lower-limit WIM-to-total Halpha luminosity fractions of 25 - 50% over the whole disk. Furthermore, they show that these fractions are roughly constant as a function of radius for the galaxies in their samples despite the decreasing star-formation rate per unit area. In their very sensitive observations, Zurita et al. (2000) observe that enhanced diffuse emission near H ii regions supports that LyC flux is leaking from these sites. This impression persists despite the fact that the extremely high contrast and sharp boundary between an H ii region and the "background" suggests that the ionized region should be photon bounded (i.e., the flux has been completely absorbed).

Zurita et al. (2002) take this notion one step further and attempt to model the DIG of NGC 157 beginning with the actual H ii region distribution and luminosities as well as the actual H i distribution based on 21 cm observations. They are able to examine a variety of escape fraction scenarios and compare the results to their deep Halpha observations. While the results do not quite match the resolution of their optical observations due to limitations in the 21 cm data, the general agreement lends additional support to a "leaky" H ii region hypothesis.

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