Are fibres in molecular cloud filaments real objects?

We analyse the morphology and kinematics of dense filamentary structures produced in a numerical simulation of a star-forming cloud of 1.4 x 10(4) M-circle dot evolving under their own self-gravity in a magnetized media. This study is motivated by recent observations of velocity-coherent substructures ('fibres') in star-forming filaments. We find such 'fibres'ubiquitously in our simulated filament. We found that a fibre in one projection is not necessarily a fibre in another projection, and thus, caution should be taken into account when considering them as real objects. We found that only the densest parts of the filament (similar to 30 per cent of the densest volume, which contains similar to 70 per cent of the mass) belongs to fibres in two projections. Moreover, it is quite common that they are formed by separated density enhancements superposed along the line of sight. Observations of fibres can yield insight into the level of turbulent substructure driven by gravity, but care should be taken in interpreting the results given the problem of line-of-sight superposition. We also studied the morphology and kinematics of the 3D skeleton (spine), finding that subfilaments accrete structured material mainly along the magnetic field lines, which are preferentially perpendicular to the skeleton. The magnetic field is at the same time dragged by the velocity field due to the gravitational collapse.