The Herschel M33 extended survey (HerM33es): PACS spectroscopy of the
  star forming region BCLMP 302
  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.
