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

B. Mookerjea*, C. Kramer, C. Buchbender, M. Boquien, S. Verley, M. Relano, G. Quintana-Lacaci, S. Aalto, J. Braine, D. Calzetti, F. Combes, S. Garcia-Burillo, P. Gratier, C. Henkel, F. Israel, S. Lord, T. Nikola, M. Roellig, G. Stacey, F. S. TabatabaeiF. van der Tak, Paul P. van der Werf

*Corresponding author for this work

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Context. The emission line of [CII] at 158 mu m 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 [CII] 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 [CII] 158 mu m, [OI] 63 mu m, and other FIR lines in a 2' x 2' region of the northern spiral arm of M 33, centered on the HII region BCLMP302. At the peak of Ha emission, we observed in addition a velocity-resolved [CII] spectrum using HIFI. We use scatterplots to compare these data with PACS 160 mu m continuum maps, and with maps of CO and HI data, at a common resolution of 12 '' or 50 pc. Maps of Ha and 24 mu m emission observed with Spitzer are used to estimate the SFR. We created maps of the [CII] and [OI] 63 mu m emission and detected [NII] 122 mu m and [NIII] 57 mu m 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 of [CII] to the TIR as a proxy, varies between 0.07 and 1.5%, with the largest variations found outside the HII region.

Original languageEnglish
Article numberA152
Number of pages12
JournalAstronomy & astrophysics
Publication statusPublished - Aug-2011


  • galaxies: individual: M 33
  • photon-dominated region (PDR)
  • HII regions
  • Local Group
  • galaxies: ISM
  • galaxies: star formation
  • M33 HERM33ES

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