Vorticity of IGM Velocity Field on Large Scales

TL;DR

This study uses cosmological simulations to analyze the vorticity and turbulence in the intergalactic medium, revealing that turbulence is fully developed on scales below 3 h^{-1} Mpc and influences baryonic matter dynamics.

Contribution

It demonstrates that the IGM exhibits fully developed turbulence on small scales and introduces vorticity as a tool to study nonlinear IGM behavior and turbulence evolution.

Findings

Vorticity increases over time due to shocks and structures.

Turbulent pressure can counteract gravitational collapse.

Turbulence is fully developed on scales less than 3 h^{-1} Mpc.

Abstract

We investigate the vorticity of the IGM velocity field on large scales with cosmological hydrodynamic simulation of the concordance model of LCDM. We show that the vorticity field is significantly increasing with time as it can effectively be generated by shocks and complex structures in the IGM. Therefore, the vorticity field is an effective tool to reveal the nonlinear behavior of the IGM, especially the formation and evolution of turbulence in the IGM. We find that the vorticity field does not follow the filaments and sheets structures of underlying dark matter density field and shows highly non- Gaussian and intermittent features. The power spectrum of the vorticity field is used to measure the development of turbulence in Fourier space. We show that the relation between the power spectra of vorticity and velocity fields is perfectly in agreement with the prediction of a fully developed homogeneous and isotropic turbulence from 0.2 to 3 h^{-1} Mpc at z~0. This indicates that cosmic baryonic field is in the state of fully developed turbulence on scales less than about 3 h^{-1} Mpc. The random field of the turbulent fluid yields turbulent pressure to prevent the gravitational collapsing of the IGM. The vorticity and turbulent pressure are strong inside and even outside of high density regions. In IGM regions with 10 times mean overdensity, the turbulent pressure can be on an average equivalent to the thermal pressure of the baryonic gas with a temperature of 10^5 K. The fully developed turbulence would prevent the baryons in the IGM from falling into the gravitational well of dark matter halos. Moreover, turbulent pressure is dynamical and non-thermal, which makes it different from pre-heating mechanism as it does not affect the thermal state and ionizing process of hydrogen in the IGM.