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Diffuse Ionized Gas (DIG) is seen as faint emission arising from ionized Hydrogren gas outside of classical HII regions. The DIG contains most of the mass of ionized interstellar gas, some 30\% of the total atomic gas mass, and fills 20\% or more of the disk volume, making it an important component of the ISM. The DIG is characterized by an enhanced [SII]/H$\alpha$ ratio compared to discrete HII regions. Imaging in these lines helps to distinguish DIG from HII regions and provides first clues on the ionization mechanism of the gas. Early observations of DIG in external systems focused on irregular and edge-on spiral galaxies, with only one extensive study of a large ``face-on'' spiral galaxy, M31. The morphology and spatial distribution of the DIG across galactic disks can only be determinied through observations of more face-on systems.
We present here some initial results of a Ph.D. thesis project which will extend the M31 study to a sample of roughly 12 spiral galaxies, to address how the DIG properties vary with Hubble type and star formation properties, whether there is any DIG between spiral arms, what the source of ionization is, and how much energy is required to keep it ionized. Our H$\alpha$ and [SII] images of M51 and M81 clearly show the presence of DIG. The DIG contributes 20-40\% of the total H$\alpha$ emission in these galaxies, similar to previous results for the Galaxy and M31. While M81 shows enhanced [SII] emission from diffuse regions relative to HII regions, M51 seems to have much weaker [SII] emission overall.
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