The properties and ionization sources of diffuse ionized gas in spiral galaxies

Diffuse Ionized Gas (DIG) is an important component of the interstellar medium (ISM) in spiral galaxies, and its presence affects the processes of star formation and galaxy evolution. Despite its importance, many of basic properties of DIG are not known, and the source of the enormous amount of energy that keeps the gas ionized is still a matter of debate. A multi-faceted approach to determining the parameters and ionization source of the DIG has been undertaken. Sensitive Ha images of four edge-on spirals were obtained to study the morphology of DIG in galaxy halos, and to look for correlations between DIG properties and galaxy types. The nearby face-on spiral M33 was studied in detail, using deep Ha imaging as well as far ultraviolet (FUV) data from the Ultraviolet Imaging Telescope (UIT ) and the Hubble Space Telescope (HST). FUV and Halpha images of several nearby spirals were used to study the connection between massive stars and the DIG. Lastly, very deep images of two edge-on galaxies were used to look for Halpha emission from the extreme outer disks of spirals, as a test of the strength of the metagalactic radiation field. In M33, archival HST images were used to investigate individual OB stars in HII regions and the DIG. Ionizing stars were found in the DIG, and they can ionize 40% +/- 12% of the DIG in the small area of the disk of M33 that was investigated. This significantly reduces the amount of leakage from HII regions necessary to explain the DIG, and eases the problem of transporting ionizing photons through the disk. UIT and Halpha images of nine other galaxies were analyzed, and they suggest that the M33 results should also apply for other spirals. (Abstract shortened by UMI.)