The Contribution of Field OB Stars to the Ionization of the Diffuse Ionized Gas in M33

(Abridged) We present a study of the ionizing stars associated with thediffuse ionized gas (DIG) and HII regions in the nearby spiral galaxy M33. Wecompare our Schmidt H-alpha image to the far-ultraviolet (FUV, 1520A) imagefrom the Ultraviolet Imaging Telescope (UIT). The H-alpha/FUV ratio is higherin HII regions than in the DIG, suggesting an older population of ionizingstars in the DIG. When compared to models of evolving stellar populations, theN(Lyc)/FUV ratio in HII regions is consistent with a young burst, while the DIGratio resembles an older burst population, or a steady state population builtup by constant star formation. The UIT data is complimented with archival FUVand optical images of a small portion of the disk of M33 obtained with WFPC2 onHST. Using the HST FUV and optical photometry, we assign spectral types to thestars observed in DIG and HII regions. The photometry indicates that ionizingstars are present in the DIG. We compare the predicted ionizing flux with theamount required to produce the observed H-alpha emission, and find that fieldOB stars in the HST images can account for 40% +/- 12% of the ionization of theDIG, while the stars in HII regions can provide 107% +/- 26% of the H-alphaluminosity of the HII regions. We do not find any correlation between leakageof ionizing photons and H-alpha luminosity for the HII regions in these HSTfields. If stellar photons alone are responsible for ionizing the DIG, thecurrent results are consistent with no or few ionizing photons escaping fromthe galaxy.Comment: 30 pages, 7 figures, accepted for publication in Ap