2000A&A...362..119T

In order to examine the excitation and ionization mechanism of extraplanar diffuse ionized gas (DIG) we have obtained optical longslit spectra of seven edge-on spiral galaxies. In four objects the brightest emission lines can be traced out to distances of typically 1.5kpc above the disk. For NGC 1963 and NGC 3044 line ratios such as [NII]λ6583/Hα or [SII]λ6717/Hα as well as [OIII]λ5007/Hβ could be measured for the halo DIG. This allows us to discuss the DIG in the halo of these objects in the framework of diagnostic diagrams. For these two objects, the line ratios of [OIII]λ5007/Hβ decrease with increasing |z|, different from the recently reported trend in NGC 891 (Rand, 1998ApJ...501..137R). We find that emission lines from the DIG in the disks are in good agreement with photoionization models using a dilute radiation field. However, with increasing |z| these models fail to predict the measured [OI]λ6300/Hα and HeIλ5876/Hα line ratios for NGC 1963. Diagnostic diagrams reveal for NGC 1963 the need for a second ionization mechanism of the halo DIG (besides photoionization). This additional source could be shock ionization. The same diagrams demonstrate an intermediate classification for NGC 3044. Plots of [SII]/[NII] vs. emission measure reveal significant changes towards the halo and seem to trace local small scale density fluctuations of the extraplanar DIG.