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H.L. Dinerstein (U. Texas, Austin), J. Brauher, D. Dale, S. Lord (IPAC/Caltech), G. Helou (Caltech), D Hollenbach, R. Rubin (NASA/ARC), S. Malhotra (NOAO), G. Stacey (Cornell)
We report observations with the Infrared Space Observatory's Long Wavelength Spectrometer (LWS) of several positions in the disk of the luminous nearby Sc galaxy M~101 (NGC~5457), including the galaxy nucleus and three giant spiral-arm H II regions: NGC~5447, NGC~5461, and NGC~5471. These H II regions span nearly a factor of three in galactocentric distance in a galaxy which is well known for its prominent radial abundance gradient. The emission contained with the 75" ISO LWS beam at positions centered on the giant H II regions is dominated by their emission plus that of the associated photodissociation regions and giant molecular clouds, with little contribution from extended diffuse disk emission. Both the [C~II] 158 \mum and [O~I] 63 \mum lines are detected from all three H~II regions and the galaxy nucleus; [O~III] 88 \mum is detected in NGC 5447 and 5461, with [N~II] 122 \mum marginally detected or below our detection limit. The [C~II]/[O~I] line ratios are indicative of warm, dense gas, as expected if the fine-structure lines originate from the photodissociated surfaces of giant molecular clouds in these active star-forming regions. The power in the [C~II] and [O~I] lines relative to the infrared continuum is consistent with grain photoelectric heating of photodissociation regions. These two diagnostic ratios do not appear to correlate strongly with metallicity within our limited sample. We also examine ratios of the far-infrared lines with other measured properties of the H~II regions such as CO emission; some of these ratios, for example, [C~II]/CO, are expected to be sensitive to metallicity effects.
This research was supported by NASA through data analysis grants to the ISO Key Project Team on ``The Interstellar Medium of Normal Galaxies'' and subcontracts to the institutions of the Co-Investigators.