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B. Otte, R. J. Reynolds, J. S. Gallagher (Dept. of Astronomy, Univ. of Wisc.-Madison)
In many spiral galaxies up to 40% of the H\alpha 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 (Rand 1998) explain the observed ratios. Based on results from the Wisconsin H Alpha Mapper (WHAM), Haffner et al. (1999) concluded that an additional electron heating mechanism must exist. Extra heating beyond that from photoionization would be the signature of an unrecognized process in the gaseous halos of galaxies. We present an observational test of the existence of such a heating process by observations of [O\,II]/H\beta, [N\,II]/H\alpha and [S\,II]/H\alpha in selected galaxies. Because [O\,II] \lambda3729Å\ has a high excitation energy, its behaviour provides a definitive test of the extra heating hypothesis. These observations also determine the amount of additional heating and thus constrain the possible mechanisms. Our observations provide strong evidence for an increase in temperature above the midplane.