The Herschel M 33 extended survey (HerM33es): PACS spectroscopy of the star-forming region BCLMP 302⋆
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
2 Instituto Radioastronomía Milimétrica, Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
3 University of Massachusetts, Department of Astronomy, LGRT-B 619E, Amherst, MA 01003, USA
4 Universidad de Granada, 18012 Granada, Spain
5 Department of Radio and Space Science, Onsala Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
6 Laboratoire d’Astrophysique de Bordeaux, Université Bordeaux 1, Observatoire de Bordeaux, OASU, UMR 5804, CNRS/INSU, BP 89, Floirac 33270, France
7 Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
8 Observatorio Astronómico Nacional (OAN) – Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
9 Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
10 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
11 IPAC, MS 100-22 California Institute of Technology, Pasadena, CA 91125, USA
12 Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
13 KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
14 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
15 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
16 SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ 7, UK
Received: 6 January 2011
Accepted: 21 June 2011
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.
Key words: galaxies: individual: M 33 / photon-dominated region (PDR) / HII regions / Local Group / galaxies: ISM / galaxies: star formation
© ESO, 2011