Further Spectroscopy of the Diffuse Ionized Gas in NGC 891 and Evidence   for a Secondary Source of Ionization
  Two long-slit spectra of the diffuse ionized gas in NGC 891 are presented. The first reveals variations parallel to the major axis in emission line ratios in the halo gas at z=700 pc. It is found that filaments of Halpha emission show lower values of [NII]/Halpha, [SII]/Halpha and [OI]/Halpha. Although this result is expected if the filaments represent the walls of evacuated chimneys, it merely reflects a more general correlation of these ratios with Halpha surface brightness along the slit, and may simply arise from radiation dilution effects. Much more interesting information on the nature of this gaseous halo comes from the second observation, which shows the vertical dependence of [NII]/Halpha, [SII]/Halpha, [OI]/Halpha, and [OIII]/Hbeta through the brightest region of the DIG halo. The most surprising result, in complete contradiction to models in which the DIG is ionized by massive stars in the disk, is that [OIII]/Hbeta rises with height above the plane for z>1 kpc (even as [NII]/Halpha, [SII]/Halpha, and [OI]/Halpha are rising, in line with expectations from such models). The run of [SII]/[NII] is also problematic, showing essentially no contrast with z. The [OIII] emission probably arises from shocks, turbulent mixing layers, or some other secondary source of ionization. Composite models in which the line emission comes from a mix of photo-ionized gas and shocks or turbulent mixing layers are considered in diagnostic diagrams, with the result that many aspects of the data can be explained at a qualitative level. 
Richard J. Rand