  Context. Massive stars are generally believed to form in supersonic turbulent
environment. However, recent observations have challenged this traditional
view. High spatial and spectral resolution observations of the Orion Molecular
Cloud and an infrared dark cloud G35.39 show a resolution-dependent turbulence,
and that high-mass stars are forming exclusively in subsonic to transonic cores
in those clouds. These studies demand a re-evaluation of the role of the
turbulence in massive star formation. Aims. We aim to study the turbulence in a
typical massive star-forming region G35.20-0.74 N with a sufficient spatial
resolution to resolve the thermal Jeans length, and a spectral resolution to
resolve the thermal linewidth. Methods. We use the ALMA dust continuum emission
to resolve fragmentation, JVLA 1.2 cm continuum to trace ionized gas, and JVLA
NH3 (1,1) to (7,7) inversion transition lines to trace linewidth, temperature,
and dynamics. We fit those lines and remove line broadening due to channel
width, thermal pressure, and velocity gradient to obtain a clean map of
intrinsic turbulence. Results. We find that (1) the turbulence in G35.20 is
overall supersonic, with mean and median Mach numbers 3.7 and 2.8,
respectively. (2) Mach number decreases from 6-7 at 0.1 pc scale to <3 towards
the central cores at 0.01 pc scale. (3) The central ALMA cores appear to be
decoupled form the host filament, evident by an opposite velocity gradient and
significantly reduced turbulence. Because of intense star formation activities
in G35.20 (as compared to the relatively young and quiescent IRDC G35.39), the
supersonic turbulence is likely replenished by protostellar outflows. G35.20
is, thus, representative of an evolved form of IRDC G35.39. More observations
of a sample of IRDCs are highly demanded to further investigate the role of
turbulence in initial conditions for the massive star formation.
