The initial conditions of stellar protocluster formation. I. A catalogue of Spitzer dark clouds

TL;DR

This study creates a comprehensive catalogue of over 11,000 Spitzer dark clouds in the Galaxy, revealing many previously unknown structures and their star formation activity, to better understand initial conditions for stellar cluster formation.

Contribution

It develops new techniques to identify and characterize IRDCs using Spitzer data, resulting in a large, detailed catalogue of these clouds across a significant Galactic region.

Findings

80% of IRDCs are newly identified by this catalogue.

Between 20% and 68% of IRDCs show signs of star formation activity.

The catalogue includes 11,303 clouds and 20,000 to 50,000 fragments.

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.