Abstract
Context: The emission line of CII at 158 micron is one of the strongest
cooling lines of the interstellar medium (ISM) in galaxies. Aims: Disentangling
the relative contributions of the different ISM phases to CII emission, is a
major topic of the HerM33es program, a Herschel key project to study the ISM in
the nearby spiral galaxy M33. Methods: Using PACS, we have mapped the emission
of CII 158 micron, OI 63 micron, and other FIR lines in a 2'x2' region of
the northern spiral arm of M33, centered on the HII region BCLMP302. At the
peak of H-alpha emission, we have observed in addition a velocity resolved
CII spectrum using HIFI. We use scatterplots to compare these data with PACS
160 micron continuum maps, and with maps of CO and HI data, at a common
resolution of 12 arcsec or 50 pc. Maps of H-alpha and 24 micron emission
observed with Spitzer are used to estimate the SFR. We have created maps of the
CII and OI 63 micron emission and detected NII 122 micron and NIII 57
micron at individual positions. Results: The CII line observed with HIFI is
significantly broader than that of CO, and slightly blue-shifted. In addition,
there is little spatial correlation between CII observed with PACS and CO
over the mapped region. There is even less spatial correlation between CII
and the atomic gas traced by HI. Detailed comparison of the observed
intensities towards the HII region with models of photo ionization and photon
dominated regions, confirms that a significant fraction, 20--30%, of the
observed CII emission stems from the ionized gas and not from the molecular
cloud. The gas heating efficiency, using the ratio between CII and the TIR as
a proxy, varies between 0.07 and 1.5%, with the largest variations found
outside the HII region.
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