High-mass star forming regions are typically thought to be dominated by supersonic motions .
We present combined Very Large Array and Green Bank Telescope ( VLA+GBT ) observations of NH _ { 3 } ( 1,1 ) and ( 2,2 ) in the infrared dark cloud ( IRDC ) G035.39-00.33 , tracing cold and dense gas down to scales of 0.07 pc .
We find that , in contrast to previous , similar studies of IRDCs , more than a third of the fitted ammonia spectra show subsonic non-thermal motions ( mean line width of 0.71 \mathrm { km~ { } s ^ { -1 } } ) , and the sonic Mach number distribution peaks around \mathcal { M } = 1 .
As possible observational and instrumental biases would only broaden the line profiles , our results provide strong upper limits to the actual value of \mathcal { M } , further strengthening our findings of narrow line widths .
This finding calls for a reevaluation of the role of turbulent dissipation and subsonic regions in massive-star and cluster formation .
Based on our findings in G035.39 , we further speculate that the coarser spectral resolution used in the previous VLA NH _ { 3 } studies may have inhibited the detection of subsonic turbulence in IRDCs .
The reduced turbulent support suggests that dynamically important magnetic fields of the 1 mG order would be required to support against possible gravitational collapse .
Our results offer valuable input into the theories and simulations that aim to recreate the initial conditions of high-mass star and cluster formation .