Detection of two power-law tails in the probability distribution functions of massive GMCs

We report the novel detection of complex high column density tails in the probability distribution functions (PDFs) for three high-mass star-forming regions (CepOB3, MonR2, NGC 6334), obtained from dust emission observed withHerschel. The low column density range can be fitted with a lognormal distribution. A first power-law tail starts above an extinctionof similar to 6-14. It has a slope of alpha = 1.3-2 for the rho aaEuro profile for an equivalent density distribution (spherical or cylindrical geometry), and is thus consistent with free-fall gravitational collapse. AboveA(V) similar to 40, 60, and 140, we detect an excess that can be fitted by a flatter power-law tail with alpha > 2. It correlates with the central regions of the cloud (ridges/hubs) of size similar to 1 pc and densities above 10(4) cm(-3). This excess may be caused by physical processes that slow down collapse and reduce the flow of mass towards higher densities. Possible are: (1) rotation, which introduces an angular momentum barrier, (2) increasing optical depth and weaker cooling, (3) magnetic fields, (4) geometrical effects, and (5) protostellar feedback. The excess/second power-law tail is closely linked to high-mass star-formation though it does not imply a universal column density threshold for the formation of (high-mass) stars.