Planck early results. XVII. Origin of the submillimetre excess dust emission in the Magellanic Clouds^⋆

¹ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
² Agenzia Spaziale Italiana Science Data Center, c/o ESRIN, via Galileo Galilei, Frascati, Italy
³ Astroparticule et Cosmologie, CNRS (UMR7164), Université Denis Diderot Paris 7, Bâtiment Condorcet, 10 rue A. Domon et Léonie Duquet, Paris, France
⁴ Astrophysics Group, Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK
⁵ Atacama Large Millimeter/submillimeter Array, ALMA Santiago Central Offices, Alonso de Cordova 3107, Vitacura, Casilla 763 0355, Santiago, Chile
⁶ CITA, University of Toronto, 60 St. George St., Toronto, ON M5S 3H8, Canada
⁷ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
e-mail: Jean-Philippe.Bernard@cesr.fr
⁸ California Institute of Technology, Pasadena, California, USA
⁹ DAMTP, University of Cambridge, Centre for Mathematical Science, Wilberforce Road, Cambridge CB3 0WA, UK
¹⁰ DSM/Irfu/SPP, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
¹¹ DTU Space, National Space Institute, Juliane Mariesvej 30, Copenhagen, Denmark
¹² Departamento de Física, Universidad de Oviedo, Avda. Calvo Sotelo s/n, Oviedo, Spain
¹³ Department of Astronomy and Astrophysics, University of Toronto, 50 Saint George Street, Toronto, Ontario, Canada
¹⁴ Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei, Tokyo 184-8501, Japan
¹⁵ Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
¹⁶ Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia, Canada
¹⁷ Department of Physics, Gustaf Hällströmin katu 2a, University of Helsinki, Helsinki, Finland
¹⁸ Department of Physics, Nagoya University, Chikusa-ku, Nagoya, 464-8602, Japan
¹⁹ Department of Physics, Princeton University, Princeton, New Jersey, USA
²⁰ Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, Indiana, USA
²¹ Department of Physics, University of California, Berkeley, California, USA
²² Department of Physics, University of California, One Shields Avenue, Davis, California, USA
²³ Department of Physics, University of California, Santa Barbara, California, USA
²⁴ Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, Illinois, USA
²⁵ Dipartimento di Fisica G. Galilei, Università degli Studi di Padova, via Marzolo 8, 35131 Padova, Italy
²⁶ Dipartimento di Fisica, Università La Sapienza, P. le A. Moro 2, Roma, Italy
²⁷ Dipartimento di Fisica, Università degli Studi di Milano, via Celoria, 16, Milano, Italy
²⁸ Dipartimento di Fisica, Università degli Studi di Trieste, via A. Valerio 2, Trieste, Italy
²⁹ Dipartimento di Fisica, Università di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
³⁰ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica, 1, Roma, Italy
³¹ Discovery Center, Niels Bohr Institute, Blegdamsvej 17, Copenhagen, Denmark
³² Dpto. Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³³ European Southern Observatory, ESO Vitacura, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
³⁴ European Space Agency, ESAC, Planck Science Office, Camino bajo del Castillo, s/n, Urbanización Villafranca del Castillo, Villanueva de la Cañada, Madrid, Spain
³⁵ European Space Agency, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
³⁶ Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University of Helsinki, Helsinki, Finland
³⁷ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, Catania, Italy
³⁸ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, Padova, Italy
³⁹ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, Monte Porzio Catone, Italy
⁴⁰ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, Trieste, Italy
⁴¹ INAF/IASF Bologna, via Gobetti 101, Bologna, Italy
⁴² INAF/IASF Milano, via E. Bassini 15, Milano, Italy
⁴³ INRIA, Laboratoire de Recherche en Informatique, Université Paris-Sud 11, Bâtiment 490, 91405 Orsay Cedex, France
⁴⁴ IPAG: Institut de Planétologie et d’Astrophysique de Grenoble, Université Joseph Fourier, Grenoble 1/CNRS-INSU, UMR 5274, 38041 Grenoble, France
⁴⁵ Imperial College London, Astrophysics group, Blackett Laboratory, Prince Consort Road, London, SW7 2AZ, UK
⁴⁶ Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA
⁴⁷ Institut Néel, CNRS, Université Joseph Fourier Grenoble I, 25 rue des Martyrs, Grenoble, France
⁴⁸ Institut d’Astrophysique Spatiale, CNRS (UMR8617) Université Paris-Sud 11, Bâtiment 121, Orsay, France
⁴⁹ Institut d’Astrophysique de Paris, CNRS UMR7095, Université Pierre & Marie Curie, 98bis boulevard Arago, Paris, France
⁵⁰ Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
⁵¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁵² Institute of Theoretical Astrophysics, University of Oslo, Blindern, Oslo, Norway
⁵³ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, La Laguna, Tenerife, Spain
⁵⁴ Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), Avda. de los Castros s/n, Santander, Spain
⁵⁵ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California, USA
⁵⁶ Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
⁵⁷ Kavli Institute for Cosmology Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁵⁸ LERMA, CNRS, Observatoire de Paris, 61 avenue de l’Observatoire, Paris, France
⁵⁹ Laboratoire AIM, IRFU/Service d’Astrophysique – CEA/DSM – CNRS – Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁶⁰ Laboratoire Traitement et Communication de l’Information, CNRS (UMR 5141) and Télécom ParisTech, 46 rue Barrault, 75634 Paris Cedex 13, France
⁶¹ Laboratoire de Physique Subatomique et de Cosmologie, CNRS/IN2P3, Université Joseph Fourier Grenoble I, Institut National Polytechnique de Grenoble, 53 rue des Martyrs, 38026 Grenoble Cedex, France
⁶² Laboratoire de l’Accélérateur Linéaire, Université Paris-Sud 11, CNRS/IN2P3, Orsay, France
⁶³ Lawrence Berkeley National Laboratory, Berkeley, California, USA
⁶⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
⁶⁵ MilliLab, VTT Technical Research Centre of Finland, Tietotie 3, Espoo, Finland
⁶⁶ National University of Ireland, Department of Experimental Physics, Maynooth, Co. Kildare, Ireland
⁶⁷ Niels Bohr Institute, Blegdamsvej 17, Copenhagen, Denmark
⁶⁸ Observational Cosmology, Mail Stop 367-17, California Institute of Technology, Pasadena, CA, 91125, USA
⁶⁹ Observatoire Astronomique de Strasbourg, CNRS, UMR 7550, 67000 Strasbourg, France
⁷⁰ Optical Science Laboratory, University College London, Gower Street, London, UK
⁷¹ SISSA, Astrophysics Sector, via Bonomea 265, 34136, Trieste, Italy
⁷² SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷³ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff, CF24 3AA, UK
⁷⁴ Space Sciences Laboratory, University of California, Berkeley, California, USA
⁷⁵ Spitzer Science Center, 1200 E. California Blvd., Pasadena, California, USA
⁷⁶ Stanford University, Dept of Physics, Varian Physics Bldg, 382 via Pueblo Mall, Stanford, California, USA
⁷⁷ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse Cedex 4, France
⁷⁸ Universities Space Research Association, Stratospheric Observatory for Infrared Astronomy, MS 211-3, Moffett Field, CA 94035, USA
⁷⁹ University of Granada, Departamento de Física Teórica y del Cosmos, Facultad de Ciencias, Granada, Spain
⁸⁰ University of Miami, Knight Physics Building, 1320 Campo Sano Dr., Coral Gables, Florida, USA
⁸¹ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland

Received: 8 January 2011
Accepted: 31 May 2011

Abstract

The integrated spectral energy distributions (SED) of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) appear significantly flatter than expected from dust models based on their far-infrared and radio emission. The still unexplained origin of this millimetre excess is investigated here using the Planck data. The integrated SED of the two galaxies before subtraction of the foreground (Milky Way) and background (CMB fluctuations) emission are in good agreement with previous determinations, confirming the presence of the millimetre excess. In the context of this preliminary analysis we do not propose a full multi-component fitting of the data, but instead subtract contributions unrelated to the galaxies and to dust emission. The background CMB contribution is subtracted using an internal linear combination (ILC) method performed locally around the galaxies. The foreground emission from the Milky Way is subtracted as a Galactic Hi template, and the dust emissivity is derived in a region surrounding the two galaxies and dominated by Milky Way emission. After subtraction, the remaining emission of both galaxies correlates closely with the atomic and molecular gas emission of the LMC and SMC. The millimetre excess in the LMC can be explained by CMB fluctuations, but a significant excess is still present in the SMC SED. The Planck and IRAS–IRIS data at 100 μm are combined to produce thermal dust temperature and optical depth maps of the two galaxies. The LMC temperature map shows the presence of a warm inner arm already found with the Spitzer data, but which also shows the existence of a previously unidentified cold outer arm. Several cold regions are found along this arm, some of which are associated with known molecular clouds. The dust optical depth maps are used to constrain the thermal dust emissivity power-law index (β). The average spectral index is found to be consistent with β = 1.5 and β = 1.2 below 500μm for the LMC and SMC respectively, significantly flatter than the values observed in the Milky Way. Also, there is evidence in the SMC of a further flattening of the SED in the sub-mm, unlike for the LMC where the SED remains consistent with β = 1.5. The spatial distribution of the millimetre dustexcess in the SMC follows the gas and thermal dust distribution. Different models are explored in order to fit the dust emission in the SMC. It is concluded that the millimetre excess is unlikely to be caused by very cold dust emission and that it could be due to a combination of spinning dust emission and thermal dust emission by more amorphous dust grains than those present in our Galaxy.