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Infrared-dark clouds (IRDCs) are believed to be the birthplaces of richclusters and thus contain the earliest phases of high-mass star formation. Weuse the Green Bank Telescope (GBT) and Very Large Array (VLA) maps of ammonia(NH3) in six IRDCs to measure their column density and temperature structure(Paper 1), and here, we investigate the kinematic structure and energy content.We find that IRDCs overall display organized velocity fields, with onlylocalized disruptions due to embedded star formation. The local effects seen inNH3 emission are not high velocity outflows but rather moderate (few km/s)increases in the line width that exhibit maxima near or coincident with themid-infrared emission tracing protostars. These line width enhancements couldbe the result of infall or (hidden in NH3 emission) outflow. Not only is thekinetic energy content insufficient to support the IRDCs against collapse, butalso the spatial energy distribution is inconsistent with a scenario ofturbulent cloud support. We conclude that the velocity signatures of the IRDCsin our sample are due to active collapse and fragmentation, in some casesaugmented by local feedback from stars.

Very Large Array Observations of Ammonia in Infrared-Dark Clouds II: Internal Kinematics