We present a study of ionized gas , 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 .