The Herschel^⋆ Exploitation of Local Galaxy Andromeda (HELGA)

I. Global far-infrared and sub-mm morphology

¹ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281, 9000 Gent, Belgium
e-mail: jacopo.fritz@ugent.be
² School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
³ CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁶ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁷ University of Massachusetts, Department of Astronomy, LGRT-B 619E, Amherst, MA 01003, USA
⁸ Laboratoire d’Astrophysique de Marseille, UMR 6110 CNRS, 38 rue F. Joliot-Curie, 13388 Marseille, France
⁹ Astrophysics Group, Imperial College, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
¹⁰ Center for Cosmology and the Department of Physics & Astronomy, University of California, Irvine, CA 92697, USA
¹¹ European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei Muenchen, Germany
¹² AIM, CEA/Saclay, Orme des Merisiers, bâtiment 709, 91191 Gif-sur-Yvette, France
¹³ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹⁴ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹⁵ Istituto di Fisica dello Spazio Interplanetario, INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁶ Dept. of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218, USA
¹⁷ University of the Western Cape Private Bag X17, Bellville, 7535 Cape Town, South Africa
¹⁸ Department of Physics & Astronomy, McMaster University, Hamilton, Ontario L8S 4M1, Canada
¹⁹ Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium

Received: 9 December 2011
Accepted: 30 August 2012

Abstract

Context. We have obtained Herschel images at five wavelengths from 100 to 500 μm of a  ~5.5 × 2.5 degree area centred on the local galaxy M 31 (Andromeda), our nearest neighbour spiral galaxy, as part of the Herschel guaranteed time project “HELGA”. The main goals of HELGA are to study the characteristics of the extended dust emission, focusing on larger scales than studied in previous observations of Andromeda at an increased spatial resolution, and the obscured star formation.

Aims. In this paper we present data reduction and Herschel maps, and provide a description of the far-infrared morphology, comparing it with features seen at other wavelengths.

Methods. We used high-resolution maps of the atomic hydrogen, fully covering our fields, to identify dust emission features that can be associated to M 31 with confidence, distinguishing them from emission coming from the foreground Galactic cirrus.

Results. Thanks to the very large extension of our maps we detect, for the first time at far-infrared wavelengths, three arc-like structures extending out to  ~21,  ~26 and  ~31 kpc respectively, in the south-western part of M 31. The presence of these features, hosting  ~2.2 × 10⁶ M_⊙ of dust, is safely confirmed by their detection in HI maps. Overall, we estimate a total dust mass of  ~5.8 × 10⁷ M_⊙, about 78% of which is contained in the two main ring-like structures at 10 and 15 kpc, at an average temperature of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a function of the galacto-centric distance, in agreement with the known metallicity gradient, with values ranging from 66 in the nucleus to  ~275 in the outermost region.

Conclusions. Dust in M 31 extends significantly beyond its optical radius (~21 kpc) and what was previously mapped in the far-infrared. An annular-like segment, located approximately at R₂₅, is clearly detected on both sides of the galaxy, and two other similar annular structures are undoubtedly detected on the south-west side even further out.