VORTICITY OF INTERGALACTIC MEDIUM VELOCITY FIELD ON LARGE SCALES

We investigate the vorticity of the intergalactic medium (IGM) velocity field on large scales with cosmological hydrodynamic simulation of the concordance model of ACDM. 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 filament and sheet structures of the underlying dark matter density field and shows highly non-Gaussian and intermittent features. The power spectrum of the vorticity field is then 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 in the scale range from 0.2 to about 3 h(-1) Mpc at z similar to 0. This indicates that the 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 1.0 x 10(5) K. Thus, the fully developed turbulence would prevent the baryons in the IGM from falling into the gravitational well of dark matter halos. Moreover, turbulent pressure essentially is dynamical and non-thermal, which makes it different from a pre-heating mechanism, as it does not affect the thermal state and ionizing process of hydrogen in the IGM.