[Paper Review] The Initial Conditions of Stellar Protocluster Formation: III. The Herschel counterparts of the Spitzer Dark Cloud catalogue

N. Peretto, C. Lenfestey+5 more|Research Explorer (The University of Manchester)|Feb 10, 2016

Astrophysics and Star Formation StudiesPhysics and Astronomy29 references18 citations

TL;DR

This study uses Herschel Hi-GAL 160 and 250 μm data to validate the Spitzer Dark Cloud (SDC) catalogue by identifying real infrared dark clouds (IRDCs) through H₂ column density peaks. It finds that 76% ± 19% of SDCs are real, dropping to 55% ± 12% for large clouds, resolving contamination from mid-infrared background fluctuations and establishing a reliable sample for Galactic-scale star formation studies.

ABSTRACT

Galactic plane surveys of pristine molecular clouds are key for establishing a Galactic-scale view of the earliest stages of star formation. For this reason Peretto & Fuller (2009) built an unbiased sample of IRDCs in the 10 deg < |l| < 65 deg, |b|<1 deg region of the Galactic plane using Spitzer 8micron extinction. However, in absorption studies, intrinsic fluctuations in the mid-infrared background can be mis-interpreted as foreground clouds. The main goal of the study presented here is to disentangle real clouds in the Spitzer Dark Cloud (SDC) catalogue from artefacts due to fluctuations in the mid-infrared background. We constructed H_2 column density maps at ~18" resolution using the 160micron and 250micron 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 better insight into the limitations of our automated identification scheme. 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 arcminute. The contamination of the PF09 catalogue by large spurious sources reflect the large uncertainties associated to the construction of the 8micron background emission, a key stage towards the identification of 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.

Motivation & Objective

• To distinguish real infrared dark clouds (IRDCs) from spurious sources in the Spitzer Dark Cloud (SDC) catalogue caused by mid-infrared background fluctuations.
• To assess the reliability of the PF09 SDC catalogue for Galactic-scale studies of star formation initial conditions.
• To identify real SDCs using Herschel Hi-GAL 160 and 250 μm data to construct high-resolution H₂ column density maps.
• To quantify the contamination rate of the SDC catalogue due to background emission fluctuations, especially for large angular-scale features.
• To establish a Herschel-confirmed SDC sample that probes a unique range of cloud properties, including compact and low-column-density systems.

Proposed method

• Constructed H₂ column density maps at ~18″ resolution using the 160 μm/250 μm brightness ratio from Herschel Hi-GAL survey data.
• Developed an automated detection scheme to identify SDCs by associating them with peaks in the Herschel column density maps.
• Performed detection simulations and visual inspection of a sub-sample to assess limitations of the automated identification method.
• Accounted for beam dilution and background fluctuations in the detection algorithm, particularly affecting small and large clouds.
• Compared the Herschel-confirmed SDC sample with BGPS and ATLASGAL surveys to evaluate the uniqueness of the SDC cloud property distribution.
• Used a 40″ aperture flux measurement and infrared contrast analysis to cross-validate associations with BGPS sources.

Experimental results

Research questions

• RQ1 What fraction of the Spitzer Dark Cloud (SDC) catalogue consists of real, physically associated clouds versus spurious detections from mid-infrared background fluctuations?
• RQ2 How does the reliability of SDC detection vary with angular size, particularly for clouds larger than ~1 arcminute?
• RQ3 To what extent do Herschel Hi-GAL column density peaks confirm the presence of real IRDCs in the PF09 catalogue?
• RQ4 How does the Herschel-confirmed SDC sample compare in terms of cloud properties (size, column density) to other major sub-millimetre surveys like BGPS and ATLASGAL?
• RQ5 Why do previous studies report lower SDC association rates with BGPS sources, and what role do source morphology and beam effects play?

Key findings

• 76% ± 19% of the SDCs in the PF09 catalogue are confirmed as real clouds based on Herschel column density peaks.
• For clouds with angular diameters larger than ~1 arcminute, the real cloud fraction drops to 55% ± 12%, indicating higher contamination from background fluctuations.
• The contamination of large SDCs is primarily due to uncertainties in the 8 μm background emission used to define the clouds in the original PF09 catalogue.
• The Herschel-confirmed SDC sample probes a unique range of cloud properties, including more compact and lower column density systems than BGPS and ATLASGAL.
• Discrepancies in BGPS source associations are largely due to beam dilution and elongated source morphology, where SDCs at the tips of extended BGPS sources are missed by centroid-based matching.
• Low-infrared-contrast BGPS sources without SDC associations are typically low-column-density IRDCs with large beam filling factors, often isolated or in low-density clump outskirts.