Far-Ultraviolet and Hα Imaging of Nearby Spiral Galaxies: The OB Stellar Population in the Diffuse Ionized Gas

We have compared Hα 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 Hα images were obtained using various ground-based telescopes. In all of the galaxies, the F_Hα/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 α=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 the IMF slope is actually steeper than α=2.35. We compared the F_Hα/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_Hα/F_UIT ratio is correlated with Hα 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_Hα/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 interarm regions.