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Aims: Understanding the fragmentation and collapse properties of the densegas during the onset of high-mass star formation. Methods: We observed themassive (~800M_sun) starless gas clump IRDC18310-4 with the Plateau de BureInterferometer (PdBI) at sub-arcsecond resolution in the 1.07mm continuumandN2H+(3-2) line emission. Results: Zooming from a single-dish low-resolutionmap to previous 3mm PdBI data, and now the new 1.07mm continuum observations,the sub-structures hierarchically fragment on the increasingly smaller spatialscales. While the fragment separations may still be roughly consistent withpure thermal Jeans fragmentation, the derived core masses are almost two ordersof magnitude larger than the typical Jeans mass at the given densities andtemperatures. However, the data can be reconciled with models usingnon-homogeneous initial density structures, turbulence and/or magnetic fields.While most sub-cores remain (far-)infrared dark even at 70mum, we identify weak70mum emission toward one core with a comparably low luminosity of ~16L_sun,re-enforcing the general youth of the region. The spectral line data alwaysexhibit multiple spectral components toward each core with comparably smallline widths for the individual components (in the 0.3 to 1.0km/s regime). Basedon single-dish C18O(2-1) data we estimate a low virial-to-gas-mass ratio<=0.25. We discuss that the likely origin of these spectral properties may bethe global collapse of the original gas clump that results in multiple spectralcomponents along each line of sight. Even within this dynamic picture theindividual collapsing gas cores appear to have very low levels of internalturbulence.

Hierarchical fragmentation and collapse signatures in a high-mass starless region