The initial conditions for stellar protocluster formation

III. The Herschel counterparts of the Spitzer Dark Cloud catalogue^⋆

¹ School of Physics & Astronomy, Cardiff University, Queen’s building, The parade, Cardiff CF24 3AA, UK
e-mail: nicolas.peretto@astro.cf.ac.uk
² Jodrell Bank Centre for Astrophysics, Alan Turing building, School of Physics & Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
³ IAPS-Istituto di Astrofisica e Planetologia Spaziali, via Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
⁵ Jeremiah Horrocks Institute, University of Central Lancashire, Preston, Lancashire PR1 2HE, UK

Received: 27 July 2015
Accepted: 5 February 2016

Abstract

Context. Galactic plane surveys of pristine molecular clouds are key for establishing a Galactic-scale view of star formation. For this reason, an unbiased sample of infrared dark clouds in the 10° < | l | < 65°, | b | < 1° region of the Galactic plane was built using Spitzer 8 μm extinction. However, intrinsic fluctuations in the mid-infrared background can be misinterpreted as foreground clouds.

Aims. The main goal of this study is to disentangle real clouds in the Spitzer Dark Cloud (SDC) catalogue from artefacts due to fluctuations in the mid-infrared background.

Methods. We constructed H₂ column density maps at ~18″ resolution using the 160 μm and 250 μm data from the Herschel Galactic plane survey Hi-GAL. We also developed an automated detection scheme that confirms the existence of a SDC through its association with a peak on these Herschel column density maps. Detection simulations, along with visual inspection of a small sub-sample of SDCs, have been performed to get more insight into the limitations of our automated identification scheme.

Results. Our analysis shows that 76( ± 19)% of the catalogued SDCs are real. This fraction drops to 55( ± 12)% for clouds with angular diameters larger than ~1 arcmin. The contamination of the PF09 catalogue by large spurious sources reflects the large uncertainties associated to the construction of the 8 μm background emission, a key stage in identiying SDCs. A comparison of the Herschel confirmed SDC sample with the BGPS and ATLASGAL samples shows that SDCs probe a unique range of cloud properties, reaching down to more compact and lower column density clouds than any of these two (sub-)millimetre Galactic plane surveys.

Conclusions. Even though about half of the large SDCs are spurious sources, the vast majority of the catalogued SDCs do have a Herschel counterpart. The Herschel-confirmed sample of SDCs offers a unique opportunity to study the earliest stages of both low- and high-mass star formation across the Galaxy.