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Context: How do molecular clouds form out of the atomic phase? And what arethe relative fractions of carbon in the ionized, atomic and molecular phase?These are questions at the heart of cloud and star formation. Methods: Usingmultiple observatories from Herschel and SOFIA to APEX and the IRAM 30mtelescope, we mapped the ionized, atomic and molecular carbon ([CII]@1900GHz,[CI]@492GHz and C18O(2-1)@220GHz) at high spatial resolution (12"-25") in fouryoung massive infrared dark clouds (IRDCs). Results: The three carbon phaseswere successfully mapped in all four regions, only in one source the [CII] lineremained a non-detection. Both the molecular and atomic phases trace the densestructures well, with [CI] also tracing material at lower column densities.[CII] exhibits diverse morphologies in our sample, from compact to diffusestructures probing the cloud environment. In at least two out of the fourregions, we find kinematic signatures strongly indicating that the dense gasfilaments have formed out of a dynamically active and turbulentatomic/molecular cloud, potentially from converging gas flows. Theatomic-to-molecular carbon gas mass ratios are low between 7% and 12% with thelowest values found toward the most quiescent region. In the three regionswhere [CII] is detected, its mass is always higher by a factor of a few thanthat of the atomic carbon. The ionized carbon emission depends as well on theradiation field, however, we also find strong [CII] emission in a regionwithout significant external sources, indicating that other processes, e.g.,energetic gas flows can contribute to the [CII] excitation as well.

Carbon in different phases ([CII], [CI], and CO) in infrared dark clouds: Cloud formation signatures and carbon gas fractions