The origin of a universal filament width in molecular clouds

ABSTRACT Filamentary structures identified in far-infrared observations of molecular clouds are typically found to have full widths at half-maximum $\sim 0.1\, {\rm pc}$. 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, $\rm{\small FWHM}{_\Sigma }$, 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 $\rm{\small FWHM}{_\Sigma }$s fall in the range $0.03\, {\rm pc}\lesssim \rm{\small FWHM}{_\Sigma }\lesssim 0.3\, {\rm pc}$, 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 $\rm{\small FWHM}{_\Sigma }$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 $\rm{\small FWHM}{_\Sigma }$s is below the observed $\sim 0.1 \, {\rm pc}$.