2015A&A...584A..67B

Filamentary structures in the interstellar medium are crucial ingredients of the star formation process. They fragment to form individual star-forming cores, and at the same time they may also funnel gas toward the central gas cores, providing an additional gas reservoir. We want to resolve the length scales for filament formation and fragmentation (resolution ≤0.1pc), in particular the Jeans length and cylinder fragmentation scale. We have observed the prototypical high-mass star-forming filament IRDC18223 with the Plateau de Bure Interferometer (PdBI) in the 3.2mm continuum and N₂H⁺(1-0) line emission in a ten-field mosaic at a spatial resolution of ∼4'' (∼14000au). The dust continuum emission resolves the filament into a chain of at least 12 relatively regularly spaced cores. The mean separation between cores is ∼0.40(±0.18)pc. While this is approximately consistent with the fragmentation of an infinite, isothermal, and gravitationally bound gas cylinder, a high mass-to-length ratio of M/l≃1000M_☉/pc requires additional turbulent and/or magnetic support against radial collapse of the filament. The N₂H⁺(1-0) data reveal a velocity gradient perpendicular to the main filament. Although rotation of the filament cannot be excluded, the data are also consistent with the main filament being comprised of several velocity-coherent subfilaments. Furthermore, this velocity gradient perpendicular to the filament resembles results toward Serpens south that are interpreted as signatures of filament formation within magnetized and turbulent sheet-like structures. Lower-density gas tracers ([CI] and C¹⁸O) reveal a similar red- and blueshifted velocity structure on scales around 60'' east and west of the filament. This may tentatively be interpreted as a signature of the large-scale cloud and the smaller scale filament being kinematically coupled. We do not identify a velocity gradient along the axis of the filament. This may be due to no significant gas flows along the filamentary axis, but it may also be partly caused by a low inclination angle of the filament with respect to the plane of the sky minimizing such a signature. The IRDC18223 3.2mm continuum data are consistent with thermal fragmentation of a gravitationally bound and compressible gas cylinder. However, the high mass-to-length ratio requires additional support - most likely turbulence and/or magnetic fields - against collapse. The N₂H⁺ spectral line data indicate a kinematic origin of the filament, but we cannot conclusively differentiate whether it has formed out of (pre-existing) velocity-coherent subfilaments, whether magnetized converging gas flows, a larger-scale collapsing cloud, or even whether rotation played a significant role during filament formation.