Photoionization results in H II regions and ionized gas, which are especially familiar and photogenic in the Milky Way and Magellanic Clouds.
2.1. Nebular emission-line diagnostics
The nebular emission-line spectra offer vital diagnostics of conditions in these regions, and are especially powerful if modeled with tailored photoionization models. However, a class of "semi-empirical" line diagnostics are widely used to probe nebular parameters. For example, the parameter (Vílchez & Pagel 1988)
and [Ne III] / H (Oey et al. 2000) probe the ionizing stellar effective temperature, while the parameters (Pagel et al. 1979)
and (Vílchez & Esteban 1996; Christensen et al. 1997)
estimate the oxygen and sulfur abundances. Using observations of LMC H II regions, Oey & Shields (2000) question the reliability of S23 and show that
a superior diagnostic of sulfur abundance, can be easily estimated from optical line strengths. All of these semi-empirical diagnostics must first be similarly tested and calibrated using objects with independently constrained parameters. The Local Group offers by far the best nebular samples in which to simultaneously: obtain spectral classifications of the ionizing stars; evaluate the gas morphology relative to the ionizing stars, thereby constraining the nebular ionization parameter; and estimate the abundances. These are the three primary parameters that determine the nebular emission. Thus, having empirical constraints on photoionization models, the behavior of the emission-line diagnostics can be calibrated. The nebular emission can also be used to constrain the hot stellar atmosphere models themselves, since these NLTE, expanding atmospheres are complicated and difficult to model. Such studies have been carried out for O and WR stars in the Magellanic Clouds and the Galaxy (Oey et al. 2000; Kennicutt et al. 2000; Crowther et al. 1999).
Another important test is to evaluate the degree to which H II regions are indeed radiation-bounded, as is normally assumed; the escape of ionizing radiation is a critical question for the ionization of the diffuse, warm ionized medium (see below), as well as the use of H recombination emission as a star formation tracer. The escape of ionizing radiation from the host galaxies themselves is also vital to understanding the ionization state of the intergalactic medium (IGM) and the reionization of the early Universe. We can test whether H II regions are radiation-bounded by simply comparing the observed stellar spectral types, and thus inferred ionizing flux, with the observed nebular emission. Currently, it appears that while most nebulae are radiation-bounded, a large subset apparently are density-bounded (Oey & Kennicutt 1997; Hunter & Massey 1990).
With adequate calibrations of nebular diagnostics against Local Group objects, we can infer various physical parameters for distant, unresolved star-forming regions.