The density distribution of accreting cosmic filaments as shaped by Kelvin–Helmholtz instability

Abstract

ABSTRACT Cosmic filaments play a crucial role in galaxy evolution, transporting gas from the intergalactic medium into galaxies. However, little is known about the efficiency of this process and whether the gas is accreted in a homogenous or clumpy way. Recent observations suggest the presence of broad gas density distributions in the circumgalactic medium, which could be related to the accretion of filaments. By means of two-dimensional high-resolution hydrodynamical simulations, we explore here the evolution of cold accreting filaments flowing through the hot circumgalactic medium (CGM) of high-z galaxies. We focus on the purely adiabatic case, not including cooling, gravity, or magnetic fields. In particular, we examine the non-linear effects of Kelvin–Helmholtz instability on the development of broad gas density distributions and on the formation of cold, dense clumps. We explore a large parameter space in the filament and perturbation properties, such as filament Mach number, initial perturbation wavelength, and thickness of the interface between the filament and the halo. We find that the time-averaged density distribution of the cold gas is qualitatively consistent with a skewed lognormal probability distribution function plus an additional component in the form of a high-density tail for high Mach numbers. Our results suggest a tight correlation between the accreting velocity and the maximum densities developing in the filament, which is consistent with the variance–Mach number relation for turbulence. Therefore, cosmological accretion could be a viable mechanism to produce turbulence and broad gas density distributions within the CGM.