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We analyse column density and temperature maps derived from Herschel dustcontinuum observations of a sample of massive infrared dark clouds(G11.11-0.12, G18.82-0.28, G28.37+0.07, G28.53-0.25). We disentangle thevelocity structure of the clouds using 13CO 1-0 and 12CO 3-2 data, showing thatthese IRDCs are the densest regions in massive giant molecular clouds and notisolated features. The probability distribution function (PDF) of columndensities for all clouds have a power-law distribution over all (high) columndensities, regardless of the evolutionary stage of the cloud: G11.11-0.12,G18.82-0.28, and G28.37+0.07 contain (proto)-stars, while G28.53-0.25 shows nosigns of star formation. This is in contrast to the purely log-normal PDFsreported for near/mid-IR extinction maps. We only find a log-normaldistribution for lower column densities, if we perform PDFs of the columndensity maps of the whole GMC in which the IRDCs are embedded. By comparing thePDF slope and the radial column density profile, we attribute the power law tothe effect of large-scale gravitational collapse and to local free-fallcollapse of pre- and protostellar cores. Independent from the PDF analysis, wefind infall signatures in the spectral profiles of 12CO for G28.37+0.07 andG11.11-0.12, supporting the scenario of gravitational collapse. IRDCs are thedensest regions within GMCs, which may be the progenitors of massive stars orclusters. At least some of the IRDCs are probably the same features as ridges(high column density regions with N>1e23 cm-2 over small areas), which weredefined for nearby IR-bright GMCs. Because IRDCs are only confined to thedensest (gravity dominated) cloud regions, the PDF constructed from this kindof a clipped image does not represent the (turbulence dominated) low columndensity regime of the cloud.

Understanding star formation in molecular clouds II. Signatures of gravitational collapse of IRDCs