On the nature of star-forming filaments: II. Sub-filaments and
  velocities
  We show that hydrodynamic turbulent cloud simulations naturally produce large
filaments made up of a network of smaller and coherent sub-filaments. Such
simulations resemble observations of filaments and fibres in nearby molecular
clouds. The sub-filaments are dynamical features formed at the stagnation
points of the turbulent velocity field where shocks dissipate the turbulent
energy. They are a ubiquitous feature of the simulated clouds, which appear
from the beginning of the simulation and are not formed by gradual
fragmentation of larger filaments. Most of the sub-filaments are
gravitationally sub-critical and do not fragment into cores, however, there is
also a significant fraction of supercritical sub-filaments which break up into
star-forming cores. The sub-filaments are coherent along their length, and the
residual velocities along their spine show that they are subsonically
contracting without any ordered rotation on scales of ~0.1 pc. Accretion flows
along the sub-filaments can feed material into star forming cores embedded
within the network. The overall mass in sub-filaments and the number of
sub-filaments increases as the cloud evolves. We propose that the formation of
filaments and sub-filaments is a natural consequence of the turbulent cascade
in the complex multi-phase interstellar medium. Sub-filaments are formed by the
high wavenumber, small scale modes in the turbulent velocity field. These are
then stretched by local shear motions and gathered together by a combination of
low wavenumber modes and gravitational contraction on larger scales, and by
doing so build up the extended filaments observed in column density maps.
