A statistical study of the mass and density structure of Infrared Dark Clouds

Abstract

How and when the mass distribution of stars in the Galaxy is set is one of the main issues of modern astronomy. Here we present a statistical study of mass and density distributions of infrared dark clouds (IRDCs) and fragments within them. These regions are pristine molecular gas structures and progenitors of stars and so provide insights into the initial conditions of star formation. This study makes use of a IRDC catalogue (Peretto & Fuller 2009), the largest sample of IRDC column density maps to date, containing a total of ~11,000 IRDCs with column densities exceeding N_{H2} = 1 X10^{22} cm^{-2} and over 50,000 single peaked IRDC fragments. The large number of objects constitutes an important strength of this study, allowing detailed analysis of the completeness of the sample and so statistically robust conclusions. Using a statistical approach to assigning distances to clouds, the mass and density distributions of the clouds and the fragments within them are constructed. The mass distributions show a steepening of the slope when switching from IRDCs to fragments, in agreement with previous results of similar structures. IRDCs and fragments are divided into unbound/bound objects by assuming Larson's relation and calculating their virial parameter. IRDCs are mostly gravitationally bound, while a significant fraction of the fragments are not. The density distribution of gravitationally unbound fragments shows a steep characteristic slope. (see paper for full Abstract).