  DR21(OH) ridge, the central part of a high-mass star and cluster forming
hub-filament system, is resolved spatially and kinematically into three nearly
parallel fibers (f1, f2, and f3) with a roughly north-south orientation, using
the observations of molecular transitions of H$^{13}$CO$^+$ (1-0), N$_2$H$^+$
(1-0), and NH$_2$D (1$_{1,1}$-1$_{0,1}$) with the Combined Array for Research
in Millimeter Astronomy. These fibers are all mildly supersonic ($\sigma_{\rm
V}$ about 2 times the sound speed), having lengths around 2 pc and widths about
0.1 pc, and they entangle and conjoin in the south where the most active
high-mass star formation takes place. They all have line masses 1 - 2 orders of
magnitude higher than their low-mass counterparts and are gravitationally
unstable both radially and axially. However, only f1 exhibits high-mass star
formation all the way along the fiber, yet f2 and f3 show no signs of
significant star formation in their northern parts. A large velocity gradient
increasing from north to south is seen in f3, and can be well reproduced with a
model of free-fall motion toward the most massive and active dense core in the
region, which corroborates the global collapse of the ridge and suggests that
the disruptive effects of the tidal forces may explain the inefficiency of star
formation in f2 and f3. On larger scales, some of the lower-density, peripheral
filaments are likely to be the outer extensions of the fibers, and provide
hints on the origin of the ridge.

Astrophysics of Galaxies