We simulated a 10pc region of a turbulent molecular cloud, allowing it to evolve under self gravity for 1.25 Myr. While the global structure of the region is still dominated by its turbulent structure, the denser gas has had time to become gravitationally organized. We treated the simulation as an observer might observe the sky; converting the density to an approximate line intensity, selecting filamentary regions for further study, and analysing the line-of-sight velocity information in these regions.
Our first main finding is that velocity characteristics very similar to those observed by \citet{2013A&A...554A..55H} form naturally in such a turbulent setup. Individually bound subfilaments display approximately sonic or subsonic dispersions, while the agglomerations that make up the larger filaments have transsonic to mildly supersonic relative motions. While we will further study the detailed evolution of these structures in future work, we speculate that this velocity structure is a relic of the supersonic turbulence that is generally taken as the initial conditions of star formation. The substructured (both in space and velocity) filaments appear without the need for magnetic fields, which are the physical mechanism that immediately spring to mind when considering a filament composed of a bundle of subfilaments.
Secondly, the same velocity structures that appear in spatially coherent filaments can show up when observing a faux-filament. Based on the line-of-sight velocity information, there is not a clear case to be made that our filament B (particularly the section with \(L > 2\)pc; see Figure \ref{filvels1}) is different from filaments C1 or D1. Examining the third dimension reveals it to in fact be composed of widely separated dense regions (Figure \ref{filaments3D}). Apparent velocity coherence similar to that seen in our most linear structures is evidently not enough to diagnose a true filament. The presence of this imposter filament in our data, not merely in projected density but also in the line-of-sight velocity, highlights the need for the continued development of sophisticated filament diagnostics, using both simulation and observation and both spatial and velocity data, in order to interpret not only existing observations of star forming filaments but also the imminent onslaught of data from ALMA.