We have compared H \alpha and far ultraviolet ( FUV ) images of 10 nearby spirals , with the goal of understanding the contribution of field OB stars to the ionization of the diffuse ionized gas ( DIG ) in spiral galaxies .
The FUV images were obtained by the Ultraviolet Imaging Telescope ( UIT ) and the H \alpha images were obtained using various ground-based telescopes .
In all of the galaxies , the F _ { H \alpha } /F _ { UIT } flux ratio is lower in the DIG than in the H ii regions .
This is likely an indication that the mean spectral type for OB stars in the field is later than that in H ii regions .
Comparison of the N _ { Lyc } / L _ { UIT } ratio with models of evolving stellar populations shows that the stellar population in the DIG is consistent with either an older single burst population or a steady state model with constant star formation and an initial mass function ( IMF ) slope steeper than \alpha =2.35 .
The steady state model is probably a more realistic representation of the stellar population outside of H ii regions .
The steep IMF slope simulates the steep present-day mass function slope expected for field OB stars , and does not necessarily indicate that IMF slope is actually steeper than \alpha =2.35 .
We compared the F _ { H \alpha } /F _ { UIT } ratio in the DIG of these galaxies with that in M33 , in which the field OB stellar population has previously been investigated using Hubble Space Telescope images .
If the mean spectral types of stars in H ii regions and in the DIG are the same as in M33 , and the difference in extinction between DIG and H ii regions is constant among galaxies , then the analysis suggests that field stars are important sources of ionization in most galaxies , and may be the dominant source in some galaxies .
The F _ { H \alpha } /F _ { UIT } ratio is correlated with H \alpha surface brightness in both DIG and H ii regions , although there is a large scatter in faint H ii regions , which may be due to undersampling the IMF in regions with a low total mass of stars formed .
The F _ { H \alpha } /F _ { UIT } ratio is often highest in the centers of galaxies and in the spiral arms , which is also where the DIG is brightest .
This can be explained if the extinction is greater in these regions , or if the fraction of DIG ionized by leakage is lower in the inter-arm regions .