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S. J. Unger (Queen Mary College, University of London, UK), J. L. Higdon, J. M. van der Hulst (Kapteyn Astronomical Institute, Groningen, NL)
We present ISO Long Wavelength Spectrometer (ISO-LWS) FIR spectra of M33, a late-type SA galaxy with diffuse spiral arms. We observe seven fine structure lines (43 - 197 um) in the nucleus and six star forming regions. The total flux in the far-infrared lines is ~ 1 (2) % of the total FIR flux at the nucleus (and at NGC 604 the brightest HII region observed). The [CII] line flux is ~ 0.6 (0.4) % FIR and the [OI] line flux is ~ 0.2 (0.2) % FIR. Similar values are observed in more powerful starburst galaxies.
We determine the global properties of the star forming regions in terms of two components: (1) giant HII regions aka the warm ionized medium (WIM) and (2) photodissociation regions (PDRs). PDRs form the interfaces between the HII region and the associated molecular clouds. We derive the electron density, effective temperature and 'average' stellar type of the HII complexes. Our FIR results are compared with results derived from observations at shorter wavelengths. Using the Kaufman et. al. PDR code we derive the average physical properties of the PDRs i.e. gas density, incident far-UV field strength and gas temperature. Finally we compare our results for M33 with other galaxies observed with ISO.
This work is based on observations with the Infrared Space Observatory, an ESA project with instruments funded by the ESA member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. The Long Wavelength Spectrometer was build by a consortium led by Queen Mary College, UK.