The Herschel M 33 extended survey (HerM33es): PACS spectroscopy of the star-forming region BCLMP 302

Context. The emission line of [C _(II)] at 158 μm is one of the strongest cooling lines of the interstellar medium (ISM) in galaxies. Aims. Distinguishing the relative contributions of the different ISM phases to [C _(II)] emission is a major objective of the HerM33es program, a Herschel key project to study the ISM in the nearby spiral galaxy M 33. Methods. Using PACS, we have mapped the emission of [C _(II)] 158 μm, [O _I] 63 μm, and other FIR lines in a 2'×2′ region of the northern spiral arm of M 33, centered on the H_(II) region BCLMP 302. At the peak of Hα emission, we observed in addition a velocity-resolved [C _(II)] spectrum using HIFI. We use scatterplots to compare these data with PACS 160 μm continuum maps, and with maps of CO and H_I data, at a common resolution of 12″ or 50 pc. Maps of Hα and 24 μm emission observed with Spitzer are used to estimate the SFR. We created maps of the [C _(II)] and [O_ I] 63 μm emission and detected [N _(II)] 122 μm and [N _(III)] 57 μm at individual positions. Results. The [C _(II)] line observed with HIFI is significantly broader than that of CO, and slightly blue-shifted. In addition, there is little spatial correlation between [C _(II)] observed with PACS and CO over the mapped region. There is even less spatial correlation between [C _(II)] and the atomic gas traced by H _I. Detailed comparison of the observed intensities towards the H _(II) region with models of photo-ionization and photon-dominated regions, confirms that a significant fraction, 20–30%, of the observed [C _(II)] emission stems from the ionized gas and not from the molecular cloud. The gas heating efficiency, using the ratio of [C _(II)] to the TIR as a proxy, varies between 0.07 and 1.5%, with the largest variations found outside the H _(II) region