Letter to the Editor

Herschel photometric observations of the nearby low metallicity irregular galaxy NGC 6822 ^*,**

¹ CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France e-mail: maud.galametz@cea.fr
² School of Physics & Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA, UK
³ Astrophysics Group, Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
⁴ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁵ Dept. of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁶ Jet Propulsion Laboratory, CA 91109, Dept. of Astronomy, California Institute of Technology, CA 91125 Pasadena, USA
⁷ Laboratoire d'Astrophysique de Marseille, UMR6110 CNRS, 38 rue F. Joliot-Curie, 13388 Marseille, France
⁸ Instituto de Astrofísica de Canarias (IAC) and Dept. de Astrofísica, Universidad de La Laguna (ULL), La Laguna, Tenerife, Spain
⁹ Institut d'Astrophysique de Paris, UMR7095 CNRS, Univ. Pierre & Marie Curie, Boulevard Arago, 75014 Paris, France
¹⁰ Dept. of Physics & Astronomy, University of California, Irvine, CA 92697, USA
¹¹ Observational Cosmology Lab, Code 665, NASA Goddard Space Flight Center Greenbelt, MD 20771, USA
¹² Dept. of Astrophysical & Planetary Sciences, CASA CB-389, Univ. of Colorado, Boulder, CO 80309, USA
¹³ ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
¹⁴ Astronomy Centre, Department of Physics & Astronomy, Univ. of Sussex, UK
¹⁵ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
¹⁶ Dept. of Physics & Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada
¹⁷ School of Physics & Astronomy, Univ. of Nottingham, University Park, Nottingham NG7 2RD, UK
¹⁸ Istituto di Fisica dello Spazio Interplanetario, INAF, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁹ Infrared Processing & Analysis Center, California Institute of Technology, Mail Code 100-22, 770 South Wilson Av, Pasadena, CA 91125, USA
²⁰ Centre for Astrophysics Research, Univ. of Hertfordshire, College Lane, Herts AL10 9AB, UK
²¹ University of Padova, Department of Astronomy, Vicolo Osservatorio 3, 35122 Padova, Italy
²² Observatoire Astronomique de Strasbourg, UMR 7550 Univ. de Strasbourg – CNRS, 11, rue de l'Université, 67000 Strasbourg, France
²³ UK Astronomy Technology Center, Royal Observatory Edinburgh, Edinburgh, EH9 3HJ, UK
²⁴ Institut fr Astronomie, Universitt Wien, Trkenschanzstr. 17, 1180 Wien, Austria

Received: 31 March 2010
Accepted: 21 April 2010

Abstract

We present the first Herschel PACS and SPIRE images of the low-metallicity galaxy NGC 6822 observed from 70 to 500 μm and clearly resolve the H ii regions with PACS and SPIRE. We find that the ratio 250/500 is dependent on the 24 μm surface brightness in NGC 6822, which would locally link the heating processes of the coldest phases of dust in the ISM to the star formation activity. We model the SEDs of some regions H ii regions and less active regions across the galaxy and find that the SEDs of H ii regions show warmer ranges of dust temperatures. We derive very high dust masses when graphite is used in our model to describe carbon dust. Using amorphous carbon, instead, requires less dust mass to account for submm emission due to its lower emissivity properties. This indicates that SED models including Herschel constraints may require different dust properties than commonly used. The global G/D of NGC 6822 is finally estimated to be 186, using amorphous carbon.