The origin of a universal filament width in molecular clouds

Filamentary structures identified in far-infrared observations of molecular clouds are typically found to have full-widths at half-maximum $\sim\!0.1$ pc. However, the physical explanation for this phenomenon is currently uncertain. We use hydrodynamic simulations of cylindrically-symmetric converging flows to show that the full-width at half-maximum of the resulting filament's surface density profile, FWHM$_{\Sigma}$, is closely related to the location of the accretion shock, where the inflow meets the boundary of the filament. For inflow Mach Number, ${\cal M}$, between 1 and 5, filament FWHM$_{\Sigma}$s fall in the range $0.03$ pc $\lesssim$ FWHM$_{\Sigma} \lesssim 0.3$ pc, with higher ${\cal M}$ resulting in narrower filaments. A large sample of filaments, seen at different evolutionary stages and with different values of ${\cal M}$, naturally results in a peaked distribution of FWHM$_{\Sigma}$s similar in shape to that obtained from far-infrared observations of molecular clouds. However, unless the converging flows are limited to ${\cal M} \lesssim 3$, the peak of the distribution of FWHM$_{\Sigma}$s is below the observed $\sim 0.1$ pc.