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C. G. Hoopes, R. A. M. Walterbos (New Mexico State University)
The large H\alpha luminosity emitted by the diffuse ionized gas (DIG) in spirals has led to the requirement that OB stars be the dominant contributor to its ionization. A still unresolved question, however, is whether these stars are in leaky HII regions, or whether they are mainly field OB stars outside of HII regions. We present a study of the massive stars and ionized gas in M31 and M33. These two galaxies have different star formation characteristics, allowing the comparison of DIG in different environments. We analyze archival HST WFPC2 images in U, B, V, and FUV filters, along with UV images from the Ultraviolet Imaging Telescope and ground-based H\alpha images. We compare the FUV flux from stars in HII regions and DIG with the H\alpha flux, and present color-magnitude diagrams for the massive stars in both DIG and HII regions. A comparison of the UV flux emitted by resolved stars in the HST images with the UV surface brightness of the UIT images is another population age indicator, and is less dependent on extinction. We analyze these data in the context of stellar population evolutionary models to determine the age and Lyman continuum production rates of stars in the DIG.
The author(s) of this abstract have provided an email address for comments about the abstract: choopes@nmsu.edu