The WEFT project: I. The emergence of turbulence in a cosmic filament

Abstract

Much of the ordinary matter in the present-day Universe still escapes direct detection. These missing baryons are thought to reside in a warm-hot intergalactic medium (WHIM) threading the filaments of the cosmic web, whose turbulent and thermal state remains poorly constrained yet underpins any attempt to observe it. Beyond its observational stakes, how this turbulence emerges, as gravitational collapse converts ordered inflow into a disordered cascade, is a question of structure formation in its own right. We characterise the emergence of turbulence in the diffuse gas of a cosmic filament, from its assembly to the present day, in the first simulation of the Web Evolution in Filament Targeted zoom simulations (WEFT) project. The cosmological zoom-in technique is applied to a single filament, evolved from z=63 to z=0 with the moving-mesh code AREPO, reaching a median gas cell size of ~8 kpc. We trace the filament assembly, the thermodynamic state of its diffuse gas, the generation of vorticity at its accretion shocks, and the growth of turbulent motions, then quantify the intermittency of the cascade through high-order velocity structure functions and their relative scaling exponents from extended self-similarity. The filament assembles by the hierarchical merging of several proto-filaments rather than by laminar accretion. Vorticity is seeded baroclinically where the rotating strands meet their accretion shocks obliquely. The flow evolves from a supersonic, shock-dominated state, statistically close to the bifractal Burgers limit, into a developed, mildly supersonic, intermittent cascade whose longitudinal exponents lie closest to the sheet-like She-Lévêque model at z=0. Turbulence is thus an intrinsic, quantifiable property of the diffuse gas of filaments. This pathfinder run provides the turbulent input for forecasts of WHIM observability.