Infrared spectroscopy of the diffuse ionized halos of edge-on galaxies

We present a study of ionized gas, polycyclic aromatic hydrocarbons (PAHs), and H-2 emission in the halos of three edge-on galaxies, NGC 891, NGC 5775, and NGC 3044, based on 10-20 mu m Spitzer Space Telescope spectra. The [Ne III]/[Ne II] ratio, an excellent measure of radiation hardness, rises with z in the halo of NGC 891. It is also higher in the halo of NGC 5775 than in the disk. NGC 3044 presents a more confusing situation. To explain the [Ne III]/[Ne II] as well as optical line ratio behavior in NGC 891, we carry out a simple exploration of parameter space with CLOUDY, which indicates a large increase in radiation temperature with height. Illustrative examples of physical models using a Monte Carlo radiative transfer code show that the rising neon ratio may be explained by adding a vertically extended, hot stellar source to a thin disk of massive stars. However, several other sources of hard spectra may be relevant. PAH features have scale heights of 430-530 pc in NGC 891 and 720-1080 pc in NGC 5775, suggesting they can be transported by disk-halo flows. Within NGC 891 and NGC 5775, scale heights are similar for all PAHs. For NGC 891, the scale heights exceed that of 8 mu m emission, indicating a transition from more ionized to more neutral PAHs with height. Most PAH equivalent widths are higher in the halos. H(2)17.03 mu m emission with scale heights of 550-580 pc in NGC 891 and 850 pc in NGC 5775 suggests a molecular component in a surprisingly thick layer.