Planck intermediate results. XIV. Dust emission at millimetre wavelengths in the Galactic plane

¹ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/lrfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
² Aalto University Metsähovi Radio Observatory and Dept of Radio Science and Engineering, PO Box 13000, 00076 Aalto, Finland
³ African Institute for Mathematical Sciences, 6-8 Melrose road, Muizenberg, Cape Town, 7701 Rondebosch, South Africa
⁴ Agenzia Spaziale Italiana Science Data Center, via del Politecnico snc, 00133 Roma, Italy
⁵ Agenzia Spaziale Italiana, viale Liegi 26, 00198 Roma, Italy
⁶ Astrophysics Group, Cavendish Laboratory, University of Cambridge, J J Thomson avenue, Cambridge CB3 0HE, UK
⁷ Astrophysics & Cosmology Research Unit, School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, 4000 Durban, South Africa
⁸ Atacama Large Millimeter/submillimeter Array, ALMA Santiago Central Offices, Alonso de Córdova 3107, Vitacura, Casilla 763 0355, Santiago, Chile
⁹ CITA, University of Toronto, 60 St. George St., Toronto ON M5S 3H8, Canada
¹⁰ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹¹ California Institute of Technology, Pasadena, USA
¹² Centro de Estudios de Física del Cosmos de Aragón (CEFCA), plaza San Juan 1, planta 2, 44001 Teruel, Spain
¹³ Computational Cosmology Center, Lawrence Berkeley National Laboratory, Berkeley, USA
¹⁴ Consejo Superior de Investigaciones Científicas (CSIC), 28037 Madrid, Spain
¹⁵ DSM/Irfu/SPP, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
¹⁶ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹⁷ Département de Physique Théorique, Université de Genève, 24 Quai E. Ansermet, 1211Genève 4, Switzerland
¹⁸ Departamento de Física Fundamental, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain
¹⁹ Departamento de Física, Universidad de Oviedo, Avda. Calvo Sotelo s/n, 33007 Oviedo, Spain
²⁰ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²¹ Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, USA
²² Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural road, Vancouver, Canada
²³ Department of Physics and Astronomy, Dana and David Dornsife College of Letter, Arts and Sciences, University of Southern California, Los Angeles CA 90089, USA
²⁴ Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
²⁵ Department of Physics, Gustaf Hällströmin katu 2a, University of Helsinki, Helsinki, Finland
²⁶ Department of Physics, Princeton University, Princeton, USA
²⁷ Department of Physics, University of California, Santa Barbara, USA
²⁸ Department of Physics, University of Illinois at Urbana- Champaign, 1110 West Green street, Urbana, USA
²⁹ Dipartimento di Fisica e Astronomia G. Galilei, Università degli Studi di Padova, via Marzolo 8, 35131 Padova, Italy
³⁰ Dipartimento di Fisica e Scienze della Terra, Università di Ferrara, via Saragat 1, 44122 Ferrara, Italy
³¹ Dipartimento di Fisica, Università La Sapienza, P. le A. Moro 2, 00185 Roma, Italy
³² Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
³³ Dipartimento di Fisica, Università degli Studi di Trieste, via A. Valerio 2, 34127 Trieste, Italy
³⁴ Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Roma, Italy
³⁵ Discovery Center, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark
³⁶ Dpto. Astrofísica, Universidad de La Laguna (ULL), 38206, La Laguna, Tenerife, Spain
³⁷ European Southern Observatory, ESO Vitacura, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
³⁸ European Space Agency, ESAC, Planck Science Office, Camino bajo del Castillo s/n, Urbanización Villafranca del Castillo, Villanueva de la Cañada, Madrid, Spain
³⁹ European Space Agency, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
⁴⁰ Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University of Helsinki, 00014 Helsinki, Finland
⁴¹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴² INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁴³ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
⁴⁴ INAF/IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁴⁵ INAF/IASF Milano, via E. Bassini 15, 20133 Milano, Italy
⁴⁶ INFN, Sezione di Bologna, via Irnerio 46, 40126 Bologna, Italy
⁴⁷ INFN, Sezione di Roma 1, Università di Roma Sapienza, piazzale Aldo Moro 2, 00185 Roma, Italy
⁴⁸ INFN/National Institute for Nuclear Physics, via Valerio 2, 34127 Trieste, Italy
⁴⁹ IPAG: Institut de Planétologie et d’Astrophysique de Grenoble, Université Joseph Fourier, Grenoble 1/CNRS-INSU, UMR 5274 , 38041 Grenoble, France
⁵⁰ IUCAA, Post Bag 4, Ganeshkhind, Pune University Campus, 411 007 Pune, India
⁵¹ Imperial College London, Astrophysics group, Blackett Laboratory, Prince Consort road, London SW7 2AZ, UK
⁵² Infrared Processing and Analysis Center, California Institute of Technology, Pasadena CA 91125, USA
⁵³ Institut Universitaire de France, 103 Bd Saint-Michel, 75005 Paris, France
⁵⁴ Institut d’Astrophysique Spatiale, CNRS (UMR8617) Université Paris-Sud 11, Bâtiment 121, 91405 Orsay, France
⁵⁵ Institut d’Astrophysique de Paris, CNRS (UMR7095), 98 bis boulevard Arago, 75014 Paris, France
⁵⁶ Institute for Space Sciences, 077125 Bucharest-Magurale, Romania
⁵⁷ Institute of Astronomy and Astrophysics, Academia Sinica, 10617 Taipei, Taiwan
⁵⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁵⁹ Institute of Theoretical Astrophysics, University of Oslo, Blindern, 0315 Oslo, Norway
⁶⁰ Instituto de Astrofísica de Canarias, C/vía Láctea s/n, 38205, La Laguna, Tenerife, Spain
⁶¹ Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), Avda. de los Castros s/n, 39005 Santander, Spain
⁶² Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove drive, Pasadena, USA
⁶³ Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁶⁴ Kavli Institute for Cosmology Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶⁵ LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France
⁶⁶ LERMA, CNRS, Observatoire de Paris, 61 avenue de l’Observatoire, 75014 Paris, France
⁶⁷ Laboratoire AIM, IRFU/Service d’Astrophysique – CEA/DSM – CNRS – Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
⁶⁸ Laboratoire Traitement et Communication de l’Information, CNRS (UMR 5141) and Télécom ParisTech, 46 rue Barrault, 75634 Paris Cedex 13, France
⁶⁹ Laboratoire de Physique Subatomique et de Cosmologie, Université Joseph Fourier Grenoble I, CNRS/IN2P3, Institut National Polytechnique de Grenoble, 53 rue des Martyrs, 38026 Grenoble Cedex, France
⁷⁰ Laboratoire de Physique Théorique, Université Paris-Sud 11 & CNRS, Bâtiment 210, 91405 Orsay, France
⁷¹ Lawrence Berkeley National Laboratory, Berkeley, USA
⁷² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
⁷³ National University of Ireland, Department of Experimental Physics, Maynooth, Co. Kildare, Ireland
⁷⁴ Niels Bohr Institute, Blegdamsvej 17, Copenhagen, Denmark
⁷⁵ Observational Cosmology, Mail Stop 367-17, California Institute of Technology, Pasadena CA 91125, USA
⁷⁶ Optical Science Laboratory, University College London, Gower Street, London, UK
⁷⁷ SISSA, Astrophysics Sector, via Bonomea 265, 34136 Trieste, Italy
⁷⁸ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
⁷⁹ Space Sciences Laboratory, University of California, Berkeley, California, USA
⁸⁰ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz, Zelenchukskiy region, 369167 Karachai- Cherkessian Republic, Russia
⁸¹ UPMC Univ Paris 06, UMR7095, 98 bis boulevard Arago, 75014 Paris, France
⁸² Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse Cedex 4, France
⁸³ Universities Space Research Association, Stratospheric Observatory for Infrared Astronomy, MS 232-11, Moffett Field CA 94035, USA
⁸⁴ University of Granada, Departamento de Física Teórica y del Cosmos, Facultad de Ciencias, Granada, Spain
⁸⁵ Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warszawa, Poland

^* Corresponding author: M. I. R. Alves marta.alves@ias.u-psud.fr

Received: 25 July 2013
Accepted: 28 January 2014

Abstract

We use Planck HFI data combined with ancillary radio data to study the emissivity index of the interstellar dust emission in the frequency range 100–353 GHz, or 3–0.8 mm, in the Galactic plane. We analyse the region l = 20°–44° and |b| ≤ 4° where the free-free emission can be estimated from radio recombination line data. We fit the spectra at each sky pixel with a modified blackbody model and two opacity spectral indices, β_mm and β_FIR, below and above 353 GHz, respectively. We find that β_mm is smaller than β_FIR, and we detect a correlation between this low frequency power-law index and the dust optical depth at 353 GHz, τ₃₅₃. The opacity spectral index β_mm increases from about 1.54 in the more diffuse regions of the Galactic disk, |b| = 3°–4° and τ₃₅₃ ~ 5 × 10^-5, to about 1.66 in the densest regions with an optical depth of more than one order of magnitude higher. We associate this correlation with an evolution of the dust emissivity related to the fraction of molecular gas along the line of sight. This translates into β_mm ~ 1.54 for a medium that is mostly atomic and β_mm ~ 1.66 when the medium is dominated by molecular gas. We find that both the two-level system model and magnetic dipole emission by ferromagnetic particles can explain the results. These results improve our understanding of the physics of interstellar dust and lead towards a complete model of the dust spectrum of the Milky Way from far-infrared to millimetre wavelengths.