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 $\sim \text{\hspace{0.17em}⁣}0.1$ 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${}_{\mathrm{\Sigma }}$, is closely related to the location of the accretion shock, where the inflow meets the boundary of the filament. For inflow Mach Number, $\mathcal{M}$, between 1 and 5, filament FWHM${}_{\mathrm{\Sigma }}$s fall in the range $0.03$ pc $\lesssim$ FWHM${}_{\mathrm{\Sigma }}\lesssim 0.3$ pc, with higher $\mathcal{M}$ resulting in narrower filaments. A large sample of filaments, seen at different evolutionary stages and with different values of $\mathcal{M}$, naturally results in a peaked distribution of FWHM${}_{\mathrm{\Sigma }}$s similar in shape to that obtained from far-infrared observations of molecular clouds. However, unless the converging flows are limited to $\mathcal{M}\lesssim 3$, the peak of the distribution of FWHM${}_{\mathrm{\Sigma }}$s is below the observed $\sim 0.1$ pc.