Infrared Dark Clouds ( IRDCs ) are unique laboratories to study the initial conditions of high-mass star and star cluster formation .
We present high-sensitivity and high-angular resolution IRAM PdBI observations of N _ { 2 } H ^ { + } ( 1 - 0 ) towards IRDC G035.39-00.33 .
It is found that G035.39-00.33 is a highly complex environment , consisting of several mildly supersonic filaments ( \sigma _ { NT } /c _ { s } \sim 1.5 ) , separated in velocity by < 1 km s ^ { -1 } .
Where multiple spectral components are evident , moment analysis overestimates the non-thermal contribution to the line-width by a factor \sim 2 .
Large-scale velocity gradients evident in previous single-dish maps may be explained by the presence of substructure now evident in the interferometric maps .
Whilst global velocity gradients are small ( < 0.7 km s ^ { -1 } pc ^ { -1 } ) , there is evidence for dynamic processes on local scales ( \sim 1.5–2.5 km s ^ { -1 } pc ^ { -1 } ) .
Systematic trends in velocity gradient are observed towards several continuum peaks .
This suggests that the kinematics are influenced by dense ( and in some cases , starless ) cores .
These trends are interpreted as either infalling material , with accretion rates \sim ( 7 \pm 4 ) \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } , or expanding shells with momentum \sim 24 \pm 12 M _ { \odot } km s ^ { -1 } .
These observations highlight the importance of high-sensitivity and high-spectral resolution data in disentangling the complex kinematic and physical structure of massive star forming regions .