  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.
