A Statistical Study of the Mass and Density Structure of Infrared Dark Clouds

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 an IRDC catalog, the largest sample of IRDC column density maps to date, containing a total of ~11,000 IRDCs with column densities exceeding cm--2 and over 50,000 single-peaked IRDC fragments. The large number of objects constitutes an important strength of this study, allowing a 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 such as Delta *DN/ Delta *Dlog(n) n --4.0?0.5, rather independent of the range of fragment mass. However, the incompleteness limit at a number density of ~103 cm--3 does not allow us to exclude a potential lognormal density distribution. In contrast, gravitationally bound fragments show a characteristic density peak at n 104 cm--3 but the shape of the density distributions changes with the range of fragment masses. An explanation for this could be the differential dynamical evolution of the fragment density with respect to their mass as more massive fragments contract more rapidly. The IRDC properties reported here provide a representative view of the density and mass structure of dense molecular clouds before and during the earliest stages of star formation. These should serve as constraints on any theoretical or numerical model to identify the physical processes involved in the formation and evolution of structure in molecular clouds.