Letter to the Editor

Clouds, filaments, and protostars: The Herschel Hi-GAL Milky Way ^*

¹ INAF-Istituto Fisica Spazio Interplanetario, via Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: sergio.molinari@ifsi-roma.inaf.it
² STFC, Rutherford Appleton Labs, Didcot, UK
³ Center for Astrophysics and Space Astronomy (CASA), Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, USA
⁴ Department of Physics and Astronomy, University College London, London, UK
⁵ Université de Toulouse, UPS, CESR, and CNRS, UMR5187, Toulouse, France
⁶ Department of Astronomy & Astrophysics, University of Toronto, Toronto, Canada
⁷ Astrophysics Research Institute, Liverpool John Moores University, UK
⁸ Spitzer Science Center, California Institute of Technology, Pasadena, CA, USA
⁹ Department of Physics & Astronomy, University of Calgary, Canada
¹⁰ INAF - Osservatorio Astrofisico di Arcetri, Firenze, Italy
¹¹ European Southern Observatory, Garching bei Muenchen, Germany
¹² LAM, Université de Provence, Marseille, France
¹³ Institut d'Astrophysique Spatiale, Université Paris-Sud, Orsay, France
¹⁴ NASA Herschel Science Center, Caltech, Pasadena, CA, USA
¹⁵ Laboratoire AIM, CEA/DSM – INSU/CNRS – Université Paris Diderot, IRFU/SAp CEA-Saclay, 91191 Gif-sur-Yvette, France
¹⁶ Institute for Astronomy, Katholieke Universiteit Leuven, Leuven, Belgium
¹⁷ LAB/CNRS, Université de Bordeaux, BP89, 33271 Floirac Cedex, France
¹⁸ INAF - Istituto di Radioastronomia, Bologna, Italy
¹⁹ School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
²⁰ School of physics, University of New South Wales, Australia
²¹ ASI Science Data Center, 00044 Frascati (Roma), Italy
²² School of Physics and Astronomy, University of Leeds, Leeds, UK
²³ Centro de Astrobiologia, CSIC-INTA. Madrid, Spain
²⁴ Astronomy Department, UCB Berkeley, CA, USA
²⁵ Joint Astronomy Center, Hilo, Hawaii
²⁶ Radio Astronomy Lab., UCB, Berkeley, CA, USA
²⁷ Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, Canada
²⁸ Dipartimento di Fisica, Università di Roma 1 “La Sapienza”, Roma, Italy
²⁹ Dipartimento di Fisica, Università di Roma 2 “Tor Vergata”, Roma, Italy
³⁰ Department of Astrophysics, Nagoya University, Nagoya, Japan
³¹ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK
³² APC/Université Paris 7 Denis Diderot/CNRS, Bâtiment Condorcet, 10, Rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
³³ Herschel Science Centre, European Space Astronomy Centre, Villafranca del Castillo. Apartado de Correos 78, 28080 Madrid, Spain
³⁴ Jet Propulsion Laboratory, Pasadena, USA
³⁵ School of Physics and Astronomy, Cardiff University, Cardiff, UK
³⁶ National Astronomical Observatories, Chinese Academy of Sciences, Beijing, PR China
³⁷ Dipartimento di Scienza e Tecnica dell'Informazione e della Comunicazione, Università di Roma 1 “La Sapienza”, Roma, Italy
³⁸ Departement de Physique, Université Laval, Québec, Canada
³⁹ Department of Physics, University of Helsinki, Finland
⁴⁰ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
⁴¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁴² MPE-MPG, Garching bei München, Germany
⁴³ Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield, UK
⁴⁴ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
⁴⁵ I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁴⁶ Dipartimento di Fisica, Università di Napoli “Federico II”, Napoli, Italy
⁴⁷ MPIfR-MPG, Bonn, Germany
⁴⁸ Centre for Astrophysics & Planetary Science, University of Kent, Canterbury, UK
⁴⁹ Dipartimento di Fisica, Universitá del Salento, Lecce, Italy
⁵⁰ Center for Radio Astronomy, University of Calgary, Calgary, Canada
⁵¹ Max-Planck-Institut für Kernphysik, Heidelberg, Germany
⁵² INAF - Osservatorio Astrofisico di Catania, Catania, Italy
⁵³ INAF Istituto di Astrofica Spaziale e Fisica Cosmica, Bologna, Italy
⁵⁴ Department of Physics and Astronomy, The Open University, Milton Keynes, UK

Received: 31 March 2010
Accepted: 14 April 2010

Abstract

We present the first results from the science demonstration phase for the Hi-GAL survey, the Herschel key program that will map the inner Galactic plane of the Milky Way in 5 bands. We outline our data reduction strategy and present some science highlights on the two observed 2° × 2° tiles approximately centered at l = 30° and l = 59°. The two regions are extremely rich in intense and highly structured extended emission which shows a widespread organization in filaments. Source SEDs can be built for hundreds of objects in the two fields, and physical parameters can be extracted, for a good fraction of them where the distance could be estimated. The compact sources (which we will call cores' in the following) are found for the most part to be associated with the filaments, and the relationship to the local beam-averaged column density of the filament itself shows that a core seems to appear when a threshold around A_V ~ 1 is exceeded for the regions in the l = 59° field; a A_V value between 5 and 10 is found for the l = 30° field, likely due to the relatively higher distances of the sources. This outlines an exciting scenario where diffuse clouds first collapse into filaments, which later fragment to cores where the column density has reached a critical level. In spite of core L/M ratios being well in excess of a few for many sources, we find core surface densities between 0.03 and 0.5 g cm^-2. Our results are in good agreement with recent MHD numerical simulations of filaments forming from large-scale converging flows.