Synthetic observations of fibrous filaments: the problems of mapping from PPV to PPP
We present synthetic C$^{18}$O observations of fragmenting filaments
produced with the moving-mesh hydrodynamic code AREPO. In the
simulations we find that numerous continuous volume density
sub-filaments form inside the parent filament due to accretion-driven
turbulence. The synthetic observations of these simulations reveal the
existence of velocity-coherent filamentary substructures which are
similar to the 'fibres' detected in Taurus.
However, we show that while there exist filamentary substructures in
both the simulations and in the synthetic observations, these are not
the same structures. Only 50$\%$ of structures identified in
position-position-velocity (PPV) space are free from line-of-sight
confusion and form a physically continuous structure in
position-position-position (PPP) space. Those PPV structures which are
not affected by line-of-sight confusion do not correspond to the
sub-filaments seen in the simulations. Sub-filaments identified as
continuous density structures in PPP are imprinted with large velocity
gradients resulting from the formation of the parent filament, in
addition to the internal turbulence driven by accretion. This leads to
the individual sub-filaments existing over a wide range of velocities
and split into multiple velocity coherent structures due to shocks.
These results suggest that properties of velocity coherent structures do
not accurately describe the underlying substructure within a filament.
Furthermore, the condition that a filamentary structure be velocity
coherent and distinct in PPV is not a sufficient condition to guarantee
that it is a continuous and distinct structure in PPP.