Giving physical significance to the Hi-GAL data: determining the distance of cold dusty cores in the Milky Way^⋆,⋆⋆

¹ Laboratoire d’astrophysique de Marseille, CNRS/INSU - Université de Provence, 13388 Marseille Cedex 13, France
e-mail: delphine.russeil@oamp.fr
² INAF-Via Fosso del Cavaliere 100, 00133 Roma, Italy
³ School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
⁴ CNRS/INSU, Laboratoire d’Astrophysique de Bordeaux, UMR 5804, BP 89, 33271 Floirac cedex, France
⁵ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ INAF, Istituto di Radioastronomia, Bologna, Italy
⁷ Center for Astrophysics and Space Astronomy (CASA), Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, USA
⁸ Laval Univ., Observatoire du Mont Megantic, Quebec, Canada
⁹ CESR, CNRS, Toulouse, France
¹⁰ Univ. of Toronto, Canada
¹¹ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
¹² Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
¹³ Center for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Herts AL10 9AB, UK
¹⁴ MPIfR-MPG, Bonn, Germany
¹⁵ Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, Bordeaux, France

Received: 1 October 2010
Accepted: 26 November 2010

Abstract

Context. Hi-GAL, an open time key-project of the Herschel satellite, was awarded 343 hours observing time to carry out a 5-band photometric imaging survey at 70, 160, 250, 350, and 500 μm of a  |b|  ≤ 1° wide strip of the Milky Way Galactic plane in the longitude range  −70° ≤ l ≤ 70°. Two 2° × 2° fields centred at l = 30° and l = 59° have been observed with the SPIRE and PACS photometric cameras in parallel mode during the Herschel science demonstration phase (SDP). From the images, compact sources are extracted for which the distance must be established in order to determine their physical properties.

Aims. The aim of this paper is to present the distance determination strategy for the Hi-GAL compact sources. We illustrate this strategy for the two fields at l = 30° and l = 59°.

Methods. The first step to determine the distance is to establish the LSR (local standard of rest) velocity of each compact source. The kinematic distance is then determined assuming a rotation curve for our Galaxy. To resolve the distance ambiguity for sources within the solar circle, we adopt a multiwavelength approach combining extinction maps, optical, and near infrared images, and velocity information from NH₃, CO and HI data. When sources can be kinematically linked to optical H ii regions, the stellar distance of the exciting stars, when known, can be attributed to all linked sources.

Results. In the two 2° × 2° SDP fields, 2678 compact sources have been identified and listed in the band-merged catalogue. About 93% of these sources have been assigned a radial velocity and distance.

Conclusions. A multiwavelength approach is necessary to assign the correct velocity to sources (especially when CO spectra have a lot of features) and to determine the distance by solving the distance ambiguity. Also, several Hi-GAL sources seem to be in the interarm region. These sources have to be investigated with dedicated programme to be compared with sources located in the spiral arms.