Characterizing precursors to stellar clusters with Herschel^⋆,⋆⋆

¹ Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309, USA
e-mail: cara.battersby@colorado.edu
² Université de Toulouse (UPS-OMP), Institut de Recherche en Astrophysique et Planétologie, France
³ CNRS, UMR 5277, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁴ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
⁵ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
⁶ Canadian Institute for Theoretical Astrophysics, University of Toronto, Toronto, ON M5S 3H8, Canada
⁷ INAF-Instituto, Fisica Spazio Interplanetario, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁸ Laboratoire AIM, CEA/DSM – INSU/CNRS – Université Paris Diderot, IRFU/SAp CEA-Saclay, 91191 Gif-sur-Yvette, France
⁹ INAF - Osservatorio Astrofisico di Arcetri, Firenze, Italy
¹⁰ European Southern Observatory, Garching bei Muenchen, Germany
¹¹ Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield, UK

Received: 22 January 2011
Accepted: 22 September 2011

Abstract

Context. Despite their profound effect on the universe, the formation of massive stars and stellar clusters remains elusive. Recent advances in observing facilities and computing power have brought us closer to understanding this formation process. In the past decade, compelling evidence has emerged that suggests infrared dark clouds (IRDCs) may be precursors to stellar clusters. However, the usual method for identifying IRDCs is biased by the requirement that they are seen in absorption against background mid-IR emission, whereas dust continuum observations allow cold, dense pre-stellar-clusters to be identified anywhere.

Aims. We aim to understand what dust temperatures and column densities characterize and distinguish IRDCs, to explore the population of dust continuum sources that are not IRDCs, and to roughly characterize the level of star formation activity in these dust continuum sources.

Methods. We use Hi-GAL 70 to 500 μm data to identify dust continuum sources in the ℓ = 30° and ℓ = 59° Hi-GAL science demonstration phase (SDP) fields, to characterize and subtract the Galactic cirrus emission, and perform pixel-by-pixel modified blackbody fits on cirrus-subtracted Hi-GAL sources. We utilize archival Spitzer data to indicate the level of star-forming activity in each pixel, from mid-IR-dark to mid-IR-bright.

Results. We present temperature and column density maps in the Hi-GAL ℓ = 30° and ℓ = 59° SDP fields, as well as a robust algorithm for cirrus subtraction and source identification using Hi-GAL data. We report on the fraction of Hi-GAL source pixels which are mid-IR-dark, mid-IR-neutral, or mid-IR-bright in both fields. We find significant trends in column density and temperature between mid-IR-dark and mid-IR-bright pixels; mid-IR-dark pixels are about 10 K colder and have a factor of 2 higher column density on average than mid-IR-bright pixels. We find that Hi-GAL dust continuum sources span a range of evolutionary states from pre- to star-forming, and that warmer sources are associated with more star formation tracers. Additionally, there is a trend of increasing temperature with tracer type from mid-IR-dark at the coldest, to outflow/maser sources in the middle, and finally to 8 and 24 μm bright sources at the warmest. Finally, we identify five candidate IRDC-like sources on the far-side of the Galaxy. These are cold (~20 K), high column density (N(H₂)  > 10²²   cm^-2) clouds identified with Hi-GAL which, despite bright surrounding mid-IR emission, show little to no absorption at 8 μm. These are the first inner Galaxy far-side candidate IRDCs of which the authors are aware.