Letter to the Editor

Dust temperature tracing the ISRF intensity in the Galaxy ^*,**

¹ Université de Toulouse, UPS, CESR, 9 avenue du colonel Roche, 31028 Toulouse Cedex 4, France e-mail: Jean-Philippe.Bernard@cesr.fr
² CNRS, UMR5187, 31028 Toulouse, France
³ Spitzer Science Center, California Institute of Technology, CALTECH, 1200 East California Boulevard, MC 220-6, Pasadena, CA 91125, USA
⁴ Institut d'Astrophysique Spatiale, Université Paris XI, 91405 Orsay, France
⁵ Dipartimento di Fisica, Università di Roma 1 “La Sapienza”, Roma, Italy
⁶ University of Exeter, Physics Building, Stocker Road, Exeter EX4 4QL, UK
⁷ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
⁸ Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁹ Laboratoire d'Astrophysique de Marseille (UMR 6110 CNRS & Université de Provence), 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁰ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
¹¹ Max Planck Institut f"ur Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
¹² The Rutherford Appleton Laboratory, Chilton, Didcot OX11 0NL, UK
¹³ Department of Physics & Astronomy, The Open University, Milton Keynes MK7 6AA, UK
¹⁴ INAF-IFSI - via Fosso del Cavaliere 100, Rome, Italy
¹⁵ ASI Science Data Center, 00044 Frascati (Rome), Italy
¹⁶ Observatory, University of Helsinki, Finland
¹⁷ Departement de Physique, Université Laval, Québec, Canada
¹⁸ Department of Physics, 00014, University of Helsinki, Finland
¹⁹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy
²⁰ University of Puerto Rico, Rio Piedras Campus, Physics Dept., Box 23343, UPR station, San Juan, Puerto Rico, USA
²¹ Dipartimento di Fisica, Università di Roma 2 “Tor Vergata”, Rome, Italy

Received: 30 March 2010
Accepted: 26 April 2010

Abstract

New observations with Herschel allow accurate measurement of the equilibrium temperature of large dust grains heated by the interstellar radiation field (ISRF), which is critical in deriving dust column density and masses. We present temperature maps derived from the Herschel SPIRE and PACS data in two fields along the Galactic plane, obtained as part of the Hi-GAL survey during the Herschel science demonstration phase (SDP). We analyze the distribution of the dust temperature spatially, as well as along the two lines-of-sight (LOS) through the Galaxy. The zero-level offsets in the Herschel maps were established by comparison with the IRAS and Planck data at comparable wavelengths. We derive maps of the dust temperature and optical depth by adjusting a detailed model for dust emission at each pixel. The dust temperature maps show variations in the ISRF intensity and reveal the intricate mixture of the warm dust heated by massive stars and the cold filamentary structures of embedded molecular clouds. The dust optical depth at 250μm is well correlated with the gas column density, but with a significantly higher dust emissivity than in the solar neighborhood. We correlate the optical depth with 3-D cubes of the dust extinction to investigate variations in the ISRF strength and dust abundance along the line of sight through the spiral structure of the Galaxy. We show that the warmest dust along the LOS is located in the spiral arms of the Galaxy, and we quantify their respective IR contribution.