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 , NGC6334 ) , obtained from dust emission observed with Herschel .
The low column density range can be fit with a lognormal distribution .
A first power-law tail starts above an extinction ( A _ { V } ) of \sim 6–14 .
It has a slope of \alpha =1.3–2 for the \rho \propto r ^ { - \alpha } profile for an equivalent density distribution ( spherical or cylindrical geometry ) , and is thus consistent with free-fall gravitational collapse .
Above A _ { V } \sim 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 \sim 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 .