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Infrared dark clouds (IRDCs) are cold and dense reservoirs of gas potentiallyavailable to form stars. Many of these clouds are likely to be pristinestructures representing the initial conditions for star formation. The studypresented here aims to construct and analyze accurate column density and dusttemperature maps of IRDCs by using the first Herschel data from the Hi-GALgalactic plane survey. These fundamental quantities, are essential forunderstanding processes such as fragmentation in the early stages of theformation of stars in molecular clouds. We have developed a simplepixel-by-pixel SED fitting method, which accounts for the background emission.By fitting a grey-body function at each position, we recover the spatialvariations in both the dust column density and temperature within the IRDCs.This method is applied to a sample of 22 IRDCs exhibiting a range of angularsizes and peak column densities. Our analysis shows that the dust temperaturedecreases significantly within IRDCs, from background temperatures of 20-30 Kto minimum temperatures of 8-15 K within the clouds, showing that densemolecular clouds are not isothermal. Temperature gradients have most likely animportant impact on the fragmentation of IRDCs. Local temperature minima arestrongly correlated with column density peaks, which in a few cases reach NH2 =1 x 10^{23} cm^{-2}, identifying these clouds as candidate massive prestellarcores. Applying this technique to the full Hi-GAL data set will provideimportant constraints on the fragmentation and thermal properties of IRDCs, andhelp identify hundreds of massive prestellar core candidates.

Mapping the column density and dust temperature structure of IRDCs with Herschel