The Ionization Sources of the Diffuse Ionized Gas in Nearby Disk Galaxies

Erica Voges

Diffuse ionized gas (DIG) has been shown to be an important component of the interstellar medium (ISM), with its large filling factor (>20%) and a mass that makes it the most massive component of the Galactic ionized ISM. Given that it has been found to be ubiquitious in both the Galaxy and external disk galaxies, the energy source to create and maintain the DIG must necessarily be large. Massive OB stars are the only known sources with enough energy to power the DIG, and DIG is also linked morphologically to OB stars as it is brightest near bright star forming regions. However, the details of the location and spectral types of the ionizing stars, as well as the relevance of other ionizing mechanisms, are still not clear.

I present the results of three different studies aimed at exploring the ionization sources of the DIG. Optical spectroscopy of DIG in M33 and NGC 891 using the Gemini-North telescope has been obtained to compare diagnostic emission line ratios with photoionization models. The first detection of [OI]6300 was made in the DIG of M33. In M33, models in which ionizing photons leaking from HII regions are responsible for the ionization of the DIG best fit our observed line ratios. In NGC 891, we found evidence that shock ionization may need to be included along with photoionization in order to explain our observed emission line ratios. The diffuse H fraction in eight nearby galaxies was studied as a function of radius and star formation rate per unit area. We found no correlation with radius, but we did find that regions with higher star formation rates have lower diffuse fractions. Neither of these results had any dependence on galaxy type. These results have implications regarding the circumstances under which HII regions may be leaking ionizing photons and thus ionizing DIG. We also compared observed and predicted ionizing fluxes for HII regions in the Large Magellanic Cloud.

Our results imply that 56% of our sample may have excess ionizing photons available to ionize DIG. All three of these studies suggest that OB stars, both in HII regions and in the field, play a major role in creating and maintaining the DIG, and that other mechanisms, such as shocks, may also contribute to the ionization of the DIG.