[Paper Review] The initial conditions of stellar protocluster formation. I. A catalogue of Spitzer dark clouds

N. Peretto, G. A. Fuller|ORCA Online Research @Cardiff (Cardiff University)|Jun 18, 2009

Astrophysics and Star Formation StudiesPhysics and Astronomy29 references176 citations

TL;DR

This paper presents a comprehensive, high-resolution catalogue of 11,303 infrared dark clouds (IRDCs) in the Milky Way using Spitzer GLIMPSE and MIPSGAL data, identifying them via 8 μm opacity maps and a novel source extraction algorithm. The study reveals that 80% of these IRDCs were previously undetected, providing a critical resource for studying the initial conditions of stellar protocluster formation, with 20,000–50,000 embedded fragments detected and 20–68% showing active star formation at 24 μm.

ABSTRACT

The majority of stars form in clusters. Therefore a comprehensive view of star formation requires understanding the initial conditions for cluster formation. The goal of our study is to shed light on the physical properties of infrared dark clouds (IRDCs) and the role they play in the formation of stellar clusters. This article, the first of a series dedicated to the study of IRDCs, describes techniques developed to establish a complete catalogue of Spitzer IRDCs in the Galaxy. We have analysed Spitzer GLIMPSE and MIPSGAL data to identify a complete sample of IRDCs in the region of Galactic longitude and latitude 10deg < |l|<65deg and |b|<1deg. From the 8micron observations we have constructed opacity maps and used a newly developed extraction algorithm to identify structures above a column density of N_{H2} > 1x10^{22} cm^{-2}. The 24micron data are then used to characterize the star formation activity of each extracted cloud. A total of 11303 clouds have been extracted. A comparison with the existing MSX based catalogue of IRDCs shows that 80% of these Spitzer dark clouds were previously unknown. The algorithm also extracts ~ 20000 to 50000 fragments within these clouds, depending on detection threshold used. A first look at the MIPSGAL data indicates that between 20% and 68% of these IRDCs show 24micron point-like association. This new database provides an important resource for future studies aiming to understand the initial conditions of star formation in the Galaxy.

Motivation & Objective

• To establish a complete, unbiased sample of infrared dark clouds (IRDCs) in the Galactic plane for studying the initial conditions of stellar cluster formation.
• To develop and apply a new algorithm for extracting IRDCs and their internal fragments from Spitzer 8 μm opacity maps.
• To characterize the star formation activity in IRDCs using 24 μm MIPSGAL data, identifying pre-protoclusters and active protoclusters.
• To compare the new Spitzer-based IRDC catalogue with the existing MSX-based catalogue to assess completeness and discovery rate.
• To provide a foundational database for future studies on the physical properties and evolution of protocluster precursors across the Galaxy.

Proposed method

• Constructed 8 μm opacity maps from Spitzer GLIMPSE data to identify IRDCs as absorption structures against the Galactic infrared background.
• Applied a new source extraction algorithm based on contour-level tracking, using a threshold opacity (τ_thres) and step size (τ_step) to detect local peaks and define source boundaries.
• Used local minima (τ_lev) between adjacent contour levels to separate overlapping structures and define fragment extents.
• Calculated physical parameters (size, position angle, centroid) using moment of inertia matrices and eigenvalue decomposition of pixel intensity distributions.
• Converted 8 μm opacity to H₂ column density using a calibration based on known dust properties and extinction laws.
• Cross-referenced detections with MIPSGAL 24 μm data to identify point-like 24 μm associations as indicators of ongoing star formation.

Experimental results

Research questions

• RQ1 What is the completeness and reliability of the Spitzer-based IRDC catalogue compared to previous MSX-based surveys?
• RQ2 How many previously undetected IRDCs and embedded fragments can be identified using high-resolution Spitzer data?
• RQ3 What fraction of IRDCs show signs of active star formation, as indicated by 24 μm point sources?
• RQ4 What are the physical properties (size, mass surface density, morphology) of the extracted IRDCs and their internal fragments?
• RQ5 To what extent do IRDCs represent pre-protoclusters that preserve the initial conditions of cluster formation?

Key findings

• A total of 11,303 IRDCs were extracted from Spitzer GLIMPSE and MIPSGAL data in the Galactic longitude range 10° < |l| < 65° and |b| < 1°.
• Approximately 80% of the detected IRDCs were previously unknown, demonstrating a significant improvement in detection completeness over the MSX-based catalogue.
• Between 20,000 and 50,000 fragments were detected within the IRDCs, depending on the chosen detection threshold.
• Between 20% and 68% of the IRDCs show 24 μm point-like associations, indicating active star formation in a substantial fraction of the sample.
• The new catalogue provides a robust, high-resolution database of IRDCs and their internal structures, ideal for studying the initial conditions of stellar protocluster formation.
• The method successfully isolates structures based on opacity contours and local minima, enabling accurate measurement of size, position angle, and centroid for each IRDC and fragment.