Infrared Spectroscopy of Halos of Edge-on Galaxies
  We present a study of ionized gas, PAHs, and molecular hydrogen emission in
the halos of three edge-on galaxies, NGC 891, NGC 5775 and NGC 3044, based on
10-20 micron 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 micron emission, indicating a transition from more
ionized to more neutral PAHs with height. Most PAH equivalent widths are higher
in the halos. Molecular hydrogen 17.03 micron 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.
