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Session 40 - The Interstellar Medium.
Display session, Tuesday, June 09
Atlas Ballroom,

[40.08] Massive Stars and the Ionization of the Diffuse Ionized Gas in M33

C. G. Hoopes, R. A. M. Walterbos, J. L. Flynn (NMSU)

The ionization source of the diffuse ionized gas (DIG) in spiral galaxies remains a subject of debate. The energetics require that OB stars are the main source of the ionization, but if these stars are within HII regions, then a significant population of density bounded HII regions is required. A possible alternative is that massive stars outside of HII regions may ionize the DIG. We present a study of the massive stars and ionized gas in the nearby spiral galaxy M33. 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 compare with models of evolving stellar population. The UV/H\alpha ratio in the DIG is consistent with a \ge 5 Myr burst population or a steady state population. We have also constructed color-magnitude diagrams for massive stars in both environments, in order to directly compare the population of ionizing stars in DIG with that in HII regions.

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