Extraplanar emission-line gas in edge-on spiral galaxies. II. Optical spectroscopy

The results from deep long-slit spectroscopy of nine edge-on spiral galaxies with known extraplanar line emission are reported. Emission from Halpha, [N II] lambdalambda6548, 6583, and [S II] lambdalambda6716, 6731 is detected out to heights of a few kiloparsecs in all of these galaxies. Several other fainter diagnostic lines such as [O I] lambda6300, [O III] lambdalambda4959, 5007, and He I lambda5876 are also detected over a smaller scale. The relative strengths, centroids, and widths of the various emission lines provide constraints on the electron density, temperature, reddening, source(s) of ionization, and kinematics of the extraplanar gas. In all but one galaxy, photoionization by massive OB stars alone has difficulties explaining all of the line ratios in the extraplanar gas. Hybrid models that combine photoionization by OB stars and another source of ionization such as photoionization by turbulent mixing layers or shocks provide a better fit to the data. The (upper limits on the) velocity gradients measured in these galaxies are consistent with the predictions of the galactic fountain model to within the accuracy of the measurements.