Letter to the Editor

Mapping the column density and dust temperature structure of IRDCs with Herschel^*

¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester, M13 9PL, UK e-mail: nicolas.peretto@manchester.ac.uk
² Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, IRFU/Service d'Astrophysique, C.E. Saclay, Orme des merisiers, 91191 Gif-sur-Yvette, France
³ Department of Physics & Astronomy, University of Calgary, Canada
⁴ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS & Université de Provence, France
⁵ Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, USA
⁶ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁷ Centre d'Études Spatiales des Rayonnements, 9 Avenue Colonel Roche, 31022 Toulouse, France
⁸ Dipartimento di Fisica, università di Roma 2, Tor vergata, Rome, Italy
⁹ Department of Astronomy & Astrophysics, University of Toronto, Toronto, Canada
¹⁰ School of Physics & Astronomy, Cardiff University, Queens Buildings, The parade, Cardiff, CF24 3AA, UK
¹¹ INAF-Instituto Fisica Spazio Interplanetario, via Fosso del Cavaliere 100, 00133 Roma, Italy
¹² Harvard-Smithosonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 31 March 2010
Accepted: 30 April 2010

Abstract

Infrared dark clouds (IRDCs) are cold and dense reservoirs of gas potentially available to form stars. Many of these clouds are likely to be pristine structures representing the initial conditions for star formation.
The study presented here aims to construct and analyze accurate column density and dust temperature maps of IRDCs by using the first Herschel data from the Hi-GAL galactic plane survey. These fundamental quantities, are essential for understanding processes such as fragmentation in the early stages of the formation of stars in molecular clouds.
We have developed a simple pixel-by-pixel SED fitting method, which accounts for the background emission. By fitting a grey-body function at each position, we recover the spatial variations in both the dust column density and temperature within the IRDCs. This method is applied to a sample of 22 IRDCs exhibiting a range of angular sizes and peak column densities. 
Our analysis shows that the dust temperature decreases significantly within IRDCs, from background temperatures of 20–30 K to minimum temperatures of 8–15 K within the clouds, showing that dense molecular clouds are not isothermal. Temperature gradients have most likely an important impact on the fragmentation of IRDCs. Local temperature minima are strongly correlated with column density peaks, which in a few cases reach = 1×10²³ cm^-2, identifying these clouds as candidate massive prestellar cores. Applying this technique to the full Hi-GAL data set will provide important constraints on the fragmentation and thermal properties of IRDCs, and help identify hundreds of massive prestellar core candidates.