Far Ultraviolet and H-alpha Imaging of Nearby Spirals: The OB Stellar Population in the Diffuse Ionized Gas

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Abstract
(Abridged) 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 HII regions. This is likely an indication that the mean spectral type for OB stars in the field is later than that in HII 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. We compared the F_H\alpha/F_UIT ratio in the DIG of these galaxies with that in M33. If the mean spectral types of stars in HII regions and in the DIG are the same as in M33, and the difference in extinction between DIG and HII 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.

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