Self-gravitating Filament Formation from Shocked Flows: Velocity
  Gradients across Filaments
  In typical environments of star-forming clouds, converging supersonic
turbulence generates shock-compressed regions, and can create
strongly-magnetized sheet-like layers. Numerical MHD simulations show that
within these post-shock layers, dense filaments and embedded self-gravitating
cores form via gathering material along the magnetic field lines. As a result
of the preferred-direction mass collection, a velocity gradient perpendicular
to the filament major axis is a common feature seen in simulations. We show
that this prediction is in good agreement with recent observations from the
CARMA Large Area Star Formation Survey (CLASSy), from which we identified
several filaments with prominent velocity gradients perpendicular to their
major axes. Highlighting a filament from the northwest part of Serpens South,
we provide both qualitative and quantitative comparisons between simulation
results and observational data. In particular, we show that the dimensionless
ratio $C_v \equiv {\Delta v_h}^2/(GM/L)$, where $\Delta v_h$ is half of the
observed perpendicular velocity difference across a filament, and $M/L$ is the
filament's mass per unit length, can distinguish between filaments formed
purely due to turbulent compression and those formed due to gravity-induced
accretion. We conclude that the perpendicular velocity gradient observed in the
Serpens South northwest filament can be caused by gravity-induced anisotropic
accretion of material from a flattened layer. Using synthetic observations of
our simulated filaments, we also propose that a density-selection effect may
explain observed subfilaments (one filament breaking into two components in
velocity space) as reported in Dhabal et al. (2018).
