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 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).
