Synthetic C$^{18}$O observations of fibrous filaments: the problems of
  mapping from PPV to PPP
  Molecular-line observations of filaments in star-forming regions have
revealed the existence of elongated coherent features within the filaments;
these features are termed fibres. Here we caution that, since fibres are traced
in PPV space, there is no guarantee that they represent coherent features in
PPP space. We illustrate this contention using simulations of the growth of a
filament from a turbulent medium. Synthetic C$^{18}$O observations of the
simulated filaments reveal the existence of fibres very similar to the observed
ones, i.e. elongated coherent features in the resulting PPV data-cubes.
Analysis of the PPP data-cubes (i.e. 3D density fields) also reveals elongated
coherent features, which we term sub-filaments. Unfortunately there is very
poor correspondence between the fibres and the sub-filaments in the
simulations. Both fibres and sub-filaments derive from inhomogeneities in the
turbulent accretion flow onto the main filament. As a consequence, fibres are
often affected by line-of-sight confusion. Similarly, sub-filaments are often
affected by large velocity gradients, and even velocity discontinuities. These
results suggest that extreme care should be taken when using velocity coherent
features to constrain the underlying substructure within a filament.
