We present a study of the ionizing stars associated with the diffuse ionized gas ( DIG ) and HII regions in the nearby spiral galaxy M33 .
We compare our Schmidt H \alpha image to the far-ultraviolet ( FUV , 1520 Å ) image from the Ultraviolet Imaging Telescope ( UIT ) .
The H \alpha /FUV ratio is higher in HII regions than in the DIG , suggesting an older population of ionizing stars in the DIG .
Assuming ionization equilibrium , we convert the H \alpha flux to the number of Lyman continuum photons N _ { Lyc } .
When compared to models of evolving stellar populations , the N _ { Lyc } /FUV ratio in HII regions is consistent with a young burst , while the DIG ratio resembles an older burst population , or a steady state population built up by constant star formation , which is probably a more accurate description of the stellar population in the field .
The UIT data is complimented with archival FUV and optical images of a small portion of the disk of M33 obtained with WFPC2 on HST .
These images overlap low- and mid-luminosity HII regions as well as DIG , so we can investigate the stellar population in these environments .
Using the HST FUV and optical photometry , we assign spectral types to the stars observed in DIG and HII regions .
The photometry indicates that ionizing stars are present in the DIG .
We compare the predicted ionizing flux with the amount required to produce the observed H \alpha emission , and find that field OB stars in the HST images can account for 40 % \pm 12 % of the ionization of the DIG , while the stars in HII regions can provide 107 % \pm 26 % of the H \alpha luminosity of the HII regions .
Due to the limited coverage of the HST data , we can not determine if stars outside the HST fields ionize some of the DIG located in the HST fields , nor can we determine if photons from stars inside the HST fields leak out of the area covered by the HST fields .
We do not find any correlation between leakage of ionizing photons and H \alpha luminosity for the HII regions in our HST fields .
However , the HST fields do not include very luminous HII regions , and it would be worthwhile to see if there is any trend at higher luminosities .
If stellar photons alone are responsible for ionizing the DIG , the current results are consistent with no or few ionizing photons escaping from the galaxy .