Cool and warm dust emission from M 33 (HerM33es) ^⋆

¹ Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, P. Penteli, 15236 Athens, Greece
e-mail: xilouris@noa.gr
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Laboratoire d’Astrophysique de Marseille – LAM, Université d’Aix-Marseille & CNRS, UMR7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
⁴ Instituto Radioastronomia Milimetrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
⁵ Argelander Institut fr Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ Laboratoire d’Astrophysique de Bordeaux, Université Bordeaux 1, Observatoire de Bordeaux, OASU, UMR 5804, CNRS/INSU, BP 89, 33270 Floirac, France
⁷ Departamento de Física Teórica y del Cosmos, Universidad de Granada, Campus Fuentenueva, Granada, Spain
⁸ Instituto de Astronomía, UNAM, Campus Ensenada, Mexico
⁹ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
¹⁰ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
¹¹ Observatoire de Paris, LERMA, CNRS, 61 av. de l’Observatoire, 75014 Paris, France
¹² Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
¹³ Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia
¹⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia
¹⁶ Infrared Processing and Analysis Center, MS 100-22 California Institute of Technology, Pasadena, CA 91125, USA
¹⁷ Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
¹⁸ University of British Columbia Okanagan, 3333 University Way, Kelowna, BC V1V 1V7, Canada
¹⁹ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
²⁰ Joint Astronomy Centre, 660 North A’ohoku Place, University Park, Hilo, HI 96720, USA
²¹ Netherlands Organization for Scientific Research, Laan van Nieuw Oost-Indie 300, NL 2509 AC The Hague, The Netherlands
²² SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
²³ SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

Received: 27 March 2012
Accepted: 3 May 2012

Abstract

In the framework of the open-time key program “Herschel M 33 extended survey (HerM33es)”, we study the far-infrared emission from the nearby spiral galaxy M 33 in order to investigate the physical properties of the dust such as its temperature and luminosity density across the galaxy. Taking advantage of the unique wavelength coverage (100, 160, 250, 350, and 500 μm) of the Herschel Space Observatory and complementing our dataset with Spitzer-IRAC 5.8 and 8 μm and Spitzer-MIPS 24 and 70 μm data, we construct temperature and luminosity density maps by fitting two modified blackbodies of a fixed emissivity index of 1.5. We find that the “cool” dust grains are heated to temperatures of between 11 K and 28 K, with the lowest temperatures being found in the outskirts of the galaxy and the highest ones both at the center and in the bright HII regions. The infrared/submillimeter total luminosity (5–1000 μm) is estimated to be 1.9 × 109 -4.4×108+4.0×108L⊙. Fifty-nine percent of the total infrared/submillimeter luminosity of the galaxy is produced by the “cool” dust grains (~15 K), while the remaining 41% is produced by “warm” dust grains (~55 K). The ratio of the cool-to-warm dust luminosity is close to unity (within the computed uncertainties), throughout the galaxy, with the luminosity of the cool dust being slightly higher at the center than the outer parts of the galaxy. Decomposing the emission of the dust into two components (one emitted by the diffuse disk of the galaxy and one emitted by the spiral arms), we find that the fraction of the emission from the disk in the mid-infrared (24 μm) is 21%, while it gradually rises up to 57% in the submillimeter (500 μm). We find that the bulk of the luminosity comes from the spiral arm network that produces 70% of the total luminosity of the galaxy with the rest coming from the diffuse dust disk. The “cool” dust inside the disk is heated to temperatures in a narrow range between 18 K and 15 K (going from the center to the outer parts of the galaxy).