Context :

Aims : Understanding the fragmentation and collapse properties of the dense gas during the onset of high-mass star formation .

Methods : We observed the massive ( \sim 800 M _ { \odot } ) starless gas clump IRDC 18310-4 with the Plateau de Bure Interferometer ( PdBI ) at sub-arcsecond resolution in the 1.07 mm continuum and N _ { 2 } H ^ { + } ( 3–2 ) line emission .

Results : Zooming from a single-dish low-resolution map to previous 3 mm PdBI data , and now the new 1.07 mm continuum observations , the sub-structures hierarchically fragment on the increasingly smaller spatial scales .
While the fragment separations may still be roughly consistent with pure thermal Jeans fragmentation , the derived core masses are almost two orders of magnitude larger than the typical Jeans mass at the given densities and temperatures .
However , the data can be reconciled with models using non-homogeneous initial density structures , turbulence and/or magnetic fields .
While most sub-cores remain ( far- ) infrared dark even at 70 \mu m , we identify weak 70 \mu m emission toward one core with a comparably low luminosity of \sim 16 L _ { \odot } , re-enforcing the general youth of the region .
The spectral line data always exhibit multiple spectral components toward each core with comparably small line widths for the individual components ( in the 0.3 to 1.0 km s ^ { -1 } regime ) .
Based on single-dish C ^ { 18 } O ( 2–1 ) data we estimate a low virial-to-gas-mass ratio \leq 0.25 .
We discuss that the likely origin of these spectral properties may be the global collapse of the original gas clump that results in multiple spectral components along each line of sight .
Even within this dynamic picture the individual collapsing gas cores appear to have very low levels of internal turbulence .

Conclusions :