Searching for additional heating in the diffuse ionized gas in galaxies

In many spiral galaxies up to 40 percent of the Halpha luminosity arises from diffuse ionized gas (DIG). DIG forms the interface between the disk and the halo and is an important component of spirals. Photoionization, mainly by leakage from disk O stars, is the favored model for the DIG. By measuring line ratios we can constrain the ionization process. It is found that neither photoionization alone nor composite models consisting of photoionization and shock ionization explain the observed ratios.; We obtained longslit spectra of a small sample of nearly edge-on galaxies to investigate the possibility of additional heating in the halo of these objects. A rise in electron temperature with increasing distance from the midplane could explain the observed line ratios. Our spectra cover a wavelength range from the singly ionized oxygen doublet at 3727 Angstrom to the singly ionized sulfur doublet at 6716.4 Angstrom. We included the singly ionized oxygen doublet as it is a good tracer of temperature variations due to its high excitation energy.; The combination of the observed emission lines allowed us to derive electron temperatures, abundances, and ionization fractions for our sample galaxies. The changing of the underlying assumptions yielded different analysis methods. The most reasonable values for electron temperature, ionization fractions, and abundances were obtained, when the oxygen abundance was held constant along the slit. In this method, the electron temperature seemed to increase towards the halo in most galaxies of our sample. In slits parallel to the midplane of the galaxies, the electron temperature increased with decreasing emission line intensity, suggesting that variations in temperature are more generally correlated with variations in gas density.; For one galaxy, NGC 3432, we obtained spectra using the fiber optic array DensePak. The resulting Halpha velocity field matched that of neutral hydrogen. We examined the velocities of individual features in the ionized gas and concluded that active star formation is occuring in this galaxy creating filaments and shells of ionized gas extending into the halo.