We analyze HST and FUSE ultraviolet spectroscopic data for 11 sight lines passing through the infalling highvelocity cloud (HVC) Complex C. These sight lines pass through regions with H i column densities in the range N H i ¼ 10 18:1 Y10 20:1 cm À2 . From [O i/H i] abundances, we find that Complex C metallicities range from 0.09 to 0.29 Z , with a column density weighted mean of 0.13 Z . Nitrogen (N i) is underabundant by factors of (0.01Y0.07)(N/H) , significantly less than oxygen relative to solar abundances. This pattern suggests nucleosynthetic enrichment by Type II SNe, consistent with an origin in the Galactic fountain or infalling gas produced in winds from Local Group galaxies. The range of metallicity and its possible (2 ) dependence on N H i could indicate some mixing of primordial material with enriched gas from the Milky Way, but the mixing mechanism is unclear. We also investigate the significant highly ionized component of Complex C, detected in C iv, Si iv, and O vi, but not in N v. High-ion column density ratios show little variance and are consistent with shock ionization or ionization at interfaces between Complex C and a hotter surrounding medium. Evidence for the former mechanism is seen in the Mrk 876 line profiles, where the offset in line centroids between low and high ions suggests a decelerating bow shock.

Deep long-slit spectra of the diffuse ionized gas halos of the edge-on spiral galaxies NGC 4302 and UGC 10288 are presented. Emission lines are detected up to about z = 2 kpc in NGC 4302, and to nearly z = 3 kpc on the north side of UGC 10288. For both galaxies, the line ratios [N II]/Hα and [S II]/Hα increase with z in accordance with dilute photo-ionization models. Runs of [S II]/[N II], and for UGC 10288, the run of [O III]/Hα, however, are not explained by the models. Scale height determinations of their DIG halos are generally lower than those of galaxies with more prominent extraplanar DIG features.These data, along with previously presented data for NGC 5775 and NGC 891, are used to address the issue of how DIG halos are energized. Composite photo-ionization/shock models are generally better at explaining runs of line ratios in these galaxies than photoionization models alone. Models of line ratios in NGC 5775 require a greater contribution from shocks for filamentary regions than for non-filamentary regions to explain the run of [O III]/Hα. In either case, the [S II]/[N II] ratio is not well fit by the models. Composite models for UGC 10288 are successful at reproducing the run of [S II]/[N II] for all but the the highest values of [N II]/Hα; however, the run of [O III]/Hα vs. [N II]/Hα does not show any discernible trend, making it difficult to determine whether or not shocks contribute to the layer's maintenance. We also examine whether the data can be explained simply by an increase in temperature with z in a pure photo-ionization model without a secondary source of ionization. Runs of [S II]/Hα, [N II]/Hα, and [S II]/[N II] in each of the four galaxies are consistent with such an increase. However, the runs of [O III]/Hα vs. z in NGC 5775 and UGC 10288 require unusually high ionization fractions of O ++ that can not be explained without invoking a secondary ionization 1 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, Tucson, AZ source or at the very least a much higher temperature for the [O III]-emitting component than for the [S II]-and [N II]-emitting component. An increase in temperature with z is generally more successful at explaining the [O III]/Hα run in NGC 891, with the ionization fraction of O ++ remaining relatively low and nearly constant with z.

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To better understand diffuse ionized gas kinematics and halo rotation in spiral galaxies, we have developed a model in which clouds are ejected from the disk and follow ballistic trajectories through the halo. The behavior of clouds in this model has been investigated thoroughly through a parameter space search and a study of individual cloud orbits. Synthetic velocity profiles have been generated in z (height above the plane) from the models for the purpose of comparing with velocity centroid data from previously obtained long-slit spectra of the edge-on spirals NGC 891 (one slit) and NGC 5775 (two slits). In each case, a purely ballistic model is insufficient in explaining observed DIG kinematics. In the case of NGC 891, the observed vertical velocity gradient is not as steep as predicted by the model, possibly suggesting a source of coupling between disk and halo rotation or an outwardly directed pressure gradient. The ballistic model more successfully explains DIG kinematics observed in NGC 5775; however, it cannot explain the observed trend of high-z gas velocities nearly reaching the systemic velocity. Such behavior can be attributed to either an inwardly directed pressure gradient or 1 Visiting Astronomer, National Optical Astronomy Observatories, Tucson, AZ -2a possible tidal interaction with its companion, NGC 5774. In addition, the ballistic model predicts that clouds move radially outward as they cycle through the halo. The mass and energy fluxes estimated from the model suggest this radially outward gas migration leads to a redistribution of material that may significantly affect the evolution of the ISM.

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