Computer Science
Scientific paper
Dec 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000immm.proc..111h&link_type=abstract
The interstellar medium in M31 and M33. Proceedings 232. WE-Heraeus Seminar, 22-25 May 2000, Bad Honnef, Germany. Edited by El
Computer Science
Scientific paper
We have investigated the diffuse ionized gas (DIG) in M33, with the goal of understanding how the gas is ionized. We used far-ultraviolet (FUV) images obtained with the Ultraviolet Imaging Telescope (UIT) and HST to trace the OB stars in M33, in order to determine the importance of field OB stars as an ionization source. Comparing the UIT image with a ground-based Hα image, we find that the FUV/Hα ratio in the DIG resembles that of a steady state population built up with constant star formation, implying the presence of ionizing stars. Using archival WFPC2 data from HST, we investigated the individual stars in the DIG and HII regions in M33, and assigned spectral types and ionizing luminosities based on the optical and FUV photometry. We find that field OB stars can ionize about 40% of the DIG in M33, with most of the rest ionized by leakage from HII regions. We also present spectroscopy of DIG in M33, obtained with the Apache Point Observatory 3.5-meter telescope. The observed spectrum agrees well with models of photoionization in a low ionization parameter environment, with the exception of the [OIII]/Hβ ratio, which is too high to be explained by existing photoionization models. We also report on the ionization state of helium in the DIG, which probes the hardness of the ionizing radiation field.
Hoopes Charles G.
Walterbos Reinirus Antonius Maria
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