Emission-line ratios and variations in temperature and ionization state in the diffuse ionized gas of five edge-on galaxies

We present spectroscopic observations of ionized gas in the disk-halo regions of five edge-on galaxies, covering a wavelength range from [O II] 3727 Angstrom to [S II] 6716.4 Angstrom. The inclusion of the [O II] emission provides additional constraints on the properties of the diffuse ionized gas ( DIG), in particular, the origin of the observed spatial variations in the line intensity ratios. We have derived electron temperatures, ionization fractions, and abundances along the slit. Our data include slit positions both parallel and perpendicular to the galactic disks. This allowed us to examine variations in the line intensity ratios with height above the midplane, as well as with distance from the galactic centers. The observed increase in the [O II]/Halpha line ratio toward the halo seems to require an increase in electron temperature caused by a nonionizing heating mechanism. We conclude that gradients in the electron temperature can play a significant role in the observed variations in the optical emission-line ratios from extraplanar DIG.