The initial conditions of stellar protocluster formation

Context. The majority of stars form in clusters. Therefore a comprehensive view of star formation requires understanding the initial conditions for cluster formation. Aims. 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. Methods. We have analysed Spitzer GLIMPSE and MIPSGAL data to identify a complete sample of IRDCs in the region of Galactic longitude and latitude 10° < |l|< 65° and |b| < 1°. From the 8 μm observations we have constructed opacity maps and used a newly developed extraction algorithm to identify structures above a column density of $N_{\rm{H_2}}$ $\ga$ 1 $\times$ 1022 cm-2. The 24 μm data are then used to characterize the star formation activity of each extracted cloud. Results. A total of 11 303 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 ~20 000 to 50 000 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 24 μm 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