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

¹ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
e-mail: bhaswati@tifr.res.in
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

Abstract

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.