Searching for additional heating: [O II] emission in the diffuse ionized gas of NGC 891,(1) NGC 4631, and NGC 3079

We present spectroscopic data of ionized gas in the disk-halo regions of three edge-on galaxies, NGC 891, NGC 4631, and NGC 3079, covering a wavelength range from [O II] 3727 Angstrom to [S II] 6716.4 Angstrom. The inclusion of the [O II] emission provides new constraints on the properties of the diffuse ionized gas, in particular the origin of the observed spatial variations in the line-intensity ratios. We used three different methods to derive electron temperatures, abundances, and ionization fractions along the slit. The increase in the [O II]/H alpha line ratio toward the halo in all three galaxies requires an increase in either electron temperature or oxygen abundance. Keeping the oxygen abundance constant yields the most reasonable results for temperature, abundances, and ionization fractions. Since a constant oxygen abundance seems to require an increase in temperature toward the halo, we conclude that gradients in the electron temperature play a significant role in the observed variations in the optical line ratios from extraplanar diffuse ionized gas in these three spiral galaxies.