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DR21(OH) ridge, the central part of a high-mass star and cluster forminghub-filament system, is resolved spatially and kinematically into three nearlyparallel fibers (f1, f2, and f3) with a roughly north-south orientation, usingthe 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 Researchin Millimeter Astronomy. These fibers are all mildly supersonic ($\sigma_{\rmV}$ about 2 times the sound speed), having lengths around 2 pc and widths about0.1 pc, and they entangle and conjoin in the south where the most activehigh-mass star formation takes place. They all have line masses 1 - 2 orders ofmagnitude higher than their low-mass counterparts and are gravitationallyunstable both radially and axially. However, only f1 exhibits high-mass starformation all the way along the fiber, yet f2 and f3 show no signs ofsignificant star formation in their northern parts. A large velocity gradientincreasing from north to south is seen in f3, and can be well reproduced with amodel of free-fall motion toward the most massive and active dense core in theregion, which corroborates the global collapse of the ridge and suggests thatthe disruptive effects of the tidal forces may explain the inefficiency of starformation in f2 and f3. On larger scales, some of the lower-density, peripheralfilaments are likely to be the outer extensions of the fibers, and providehints on the origin of the ridge.

The DR21(OH) Trident -- Resolving the Massive Ridge into Three Entangled Fibers As the Initial Condition of Cluster Formation