Spectroscopy of diffuse ionized gas in halos of selected edge-on galaxies

In order to examine the excitation and ionization mechanism of extraplanar diffuse ionized gas (DIG) we have obtained optical longslit spectra of seve n edge-on spiral galaxies. In four objects the brightest emission lines can be traced out to distances of typically 1.5 kpc above the disk. For NGC 19 63 and NGC 3044 line ratios such as [N II] lambda 6583/H alpha or [S II] la mbda 6717/H alpha as well as [O III] lambda 5007/H beta could be measured f or the halo DIG. This allows us to discuss the DIG in the halo of these obj ects in the framework of diagnostic diagrams. For these two objects, the li ne ratios of [O III] lambda 5007/H beta decrease with increasing \z\, diffe rent from the recently reported trend in NGC 891 (Rand 1998). We find that emission lines from the DIG in the disks are in good agreement with photoio nization models using a dilute radiation field. However, with increasing \z \ these models fail to predict the measured [O I] lambda 6300/H alpha and H e I lambda 5876/H alpha line ratios for NGC 1963. Diagnostic diagrams revea l 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 [S II]/[N Ir] vs. emission measure reveal significant changes tow ards the halo and seem to trace local small scale density fluctuations of t he extraplanar DIG.