Understanding star formation in molecular clouds II. Signatures of
  gravitational collapse of IRDCs
  We analyse column density and temperature maps derived from Herschel dust
continuum 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 the
velocity structure of the clouds using 13CO 1-0 and 12CO 3-2 data, showing that
these IRDCs are the densest regions in massive giant molecular clouds and not
isolated features. The probability distribution function (PDF) of column
densities for all clouds have a power-law distribution over all (high) column
densities, 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 no
signs of star formation. This is in contrast to the purely log-normal PDFs
reported for near/mid-IR extinction maps. We only find a log-normal
distribution for lower column densities, if we perform PDFs of the column
density maps of the whole GMC in which the IRDCs are embedded. By comparing the
PDF slope and the radial column density profile, we attribute the power law to
the effect of large-scale gravitational collapse and to local free-fall
collapse of pre- and protostellar cores. Independent from the PDF analysis, we
find infall signatures in the spectral profiles of 12CO for G28.37+0.07 and
G11.11-0.12, supporting the scenario of gravitational collapse. IRDCs are the
densest regions within GMCs, which may be the progenitors of massive stars or
clusters. 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 were
defined for nearby IR-bright GMCs. Because IRDCs are only confined to the
densest (gravity dominated) cloud regions, the PDF constructed from this kind
of a clipped image does not represent the (turbulence dominated) low column
density regime of the cloud.
