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 th ree edge-on galaxies, NGC 891, NGC 4631, and NGC 3079, covering a wavelengt h 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 di ffuse ionized gas, in particular the origin of the observed spatial variati ons 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 thre e galaxies requires an increase in either electron temperature or oxygen ab undance. Keeping the oxygen abundance constant yields the most reasonable r esults for temperature, abundances, and ionization fractions. Since a const ant oxygen abundance seems to require an increase in temperature toward the halo, we conclude that gradients in the electron temperature play a signif icant role in the observed variations in the optical line ratios from extra planar diffuse ionized gas in these three spiral galaxies.