We present deep H spectroscopy toward several high-velocity clouds (HVCs), which vary in structure from compact HVCs (CHVCs) to the Magellanic Stream. The clouds range from being bright ($640 mR) to having upper limits on the order of 30-70 mR. The H measurements are discussed in relation to their H i properties, and distance constraints are given to each of the complexes based onf f esc % 6% of the ionizing photons escaping normal to the Galactic disk ( f esc % 1% 2% when averaged over solid angle). The results suggest that many HVCs and CHVCs are within a $40 kpc radius from the Galaxy and are not members of the Local Group at megaparsec distances. However, the Magellanic Stream is inconsistent with this model and needs to be explained. It has bright H emission and little [N ii] emission and appears to fall into a different category than the currently detected HVCs. This may reflect the lower metallicities of the Magellanic Clouds compared to the Galaxy, but the strength of the H emission cannot be explained solely by photoionization from the Galaxy. The interaction of the Magellanic Stream with halo gas or the presence of yet unassociated young stars may assist in ionizing the Magellanic Stream.

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The Magellanic Stream and several high-velocity clouds have now been detected in optical line emission. The observed emission measures and kinematics are most plausibly explained by photoionization due to hot, young stars in the Galactic disk. The highly favorable orientation of the Stream allows an unambiguous determination of the fraction of ionizing photons which escape the Galactic disk. We have modeled the production and f esc transport of ionizing photons through an opaque interstellar medium. Normalization to the Stream detections requires , which is in reasonable agreement with the flux required to ionize the Reynolds layer. Neither f ≈ 6% esc shock heating nor emission within a hot Galactic corona can be important in producing the observed Ha emission. If such a large escape fraction is typical of galaxies, star-forming systems dominate the extragalactic ionizing L * background. Within the context of this model, both the three-dimensional orientation of the Stream and the distances to high-velocity clouds can be determined by sensitive Ha observations.

Context. We study the λ 158 μm [C II] fine-structure line emission from star-forming regions as a function of metallicity. Aims. We have measured and mapped the [C II] emission from the very bright HII region complexes N 11 in the LMC and N 66 in the SMC; as well as the SMC H II regions N 25, N 27, N 83/N 84, and N 88 with the FIFI instrument on the Kuiper Airborne Observatory. Methods. In both LMC and SMC, the ratio of [C II] line to CO line and to the far-infrared continuum emission is much higher than seen almost anywhere else, including Milky Way star-forming regions, and whole galaxies. Results. In the low metallicity, low dust-abundance environment of the LMC and the SMC UV mean free path lengths are much greater than those in the higher-metallicity Milky Way. The increased photoelectric heating efficiencies cause significantly greater relative [C II] line emission strengths. At the same time, similar decreases in PAH abundances have the opposite effect by diminishing photoelectric heating rates. Consequently, in low-metallicity environments relative [C II] strengths are high but exhibit little further dependence on actual metallicity. Relative [C II] strengths are slightly higher in the LMC than in the SMC which has both lower dust and lower PAH abundances.