2011A&A...532A.152M

The emission line of [CII] at 158µm is one of the strongest cooling lines of the interstellar medium (ISM) in galaxies. Distinguishing the relative contributions of the different ISM phases to [CII] emission is a major objective of the HerM33es program, a Herschel key project to study the ISM in the nearby spiral galaxy M33. Using PACS, we have mapped the emission of [CII] 158µm, [OI] 63µm, 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 Hα emission, we observed in addition a velocity-resolved [CII] spectrum using HIFI. We use scatterplots to compare these data with PACS 160µm continuum maps, and with maps of CO and HI data, at a common resolution of 12" or 50pc. Maps of Hα and 24µm emission observed with Spitzer are used to estimate the SFR. We created maps of the [CII] and [OI] 63µm emission and detected [NII] 122µm and [NIII] 57µm at individual positions. 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 of [CII] to the TIR as a proxy, varies between 0.07 and 1.5%, with the largest variations found outside the HII region.