Letter to the Editor

Cool gas and dust in M 33: Results from the HERschel M 33 Extended Survey (HERM33ES)^*

¹ Laboratoire d'Astrophysique de Bordeaux, Université Bordeaux 1, Observatoire de Bordeaux, OASU, UMR 5804, CNRS/INSU, BP 89, 33270 Floirac, France e-mail: Braine@obs.u-bordeaux1.fr
² Instituto Radioastronomia Milimetrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
³ Institute of Astronomy and Astrophysics, National Observatory of Athens, P. Penteli, 15236 Athens, Greece
⁴ University of British Columbia, Okanagan, 3333 University Way, Kelowna BC V1V 1V7 Canada
⁵ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
⁶ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany, Argelander Institut fr Astronomie. Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Dept. Física Teórica y del Cosmos, Universidad de Granada, Spain
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁹ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁰ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
¹¹ Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France

¹² Observatorio Astronómico Nacional (OAN) - Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
¹³ Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia

¹⁴ Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
¹⁵ KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
¹⁶ IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France

¹⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁸ Argelander Institut fr Astronomie. Auf dem Hügel 71, 53121 Bonn, Germany 


Received: 30 March 2010
Accepted: 11 May 2010

Abstract

We present an analysis of the first space-based far-IR-submm observations of M 33, which measure the emission from the cool dust and resolve the giant molecular cloud complexes. With roughly half-solar abundances, M 33 is a first step towards young low-metallicity galaxies where the submm may be able to provide an alternative to CO mapping to measure their H₂ content. In this Letter, we measure the dust emission cross-section σ using SPIRE and recent CO and H i observations; a variation in σ is present from a near-solar neighborhood cross-section to about half-solar with the maximum being south of the nucleus. Calculating the total H column density from the measured dust temperature and cross-section, and then subtracting the H i column, yields a morphology similar to that observed in CO. The H₂/H i mass ratio decreases from about unity to well below 10% and is about 15% averaged over the optical disk. The single most important observation to reduce the potentially large systematic errors is to complete the CO mapping of M 33.