2005ApJ...625..599K

The dynamic evolution of the baryonic intergalactic medium (IGM) caused by the underlying dark matter gravity is governed by the Navier-Stokes equations in which many cooling and heating processes are involved. However, it has long been recognized that the growth mode dynamics of cosmic matter clustering can be sketched by a random force driven Burgers equation if cooling and heating are ignored. Just how well the dynamics of the IGM can be described as a Burgers fluid has not been fully investigated, probably because cooling and heating are essential for a detailed understanding of the IGM. Using IGM samples produced by a cosmological hydrodynamic simulation in which heating and cooling processes are properly accounted for, we show that the IGM velocity field in the nonlinear regime shows the features of a Burgers fluid, that is, when the Reynolds number is high, the velocity field consists of an ensemble of shocks. Consequently, (1) the IGM velocity v is generally smaller than that of dark matter; (2) for the smoothed field, the IGM velocity shows tight correlation with dark matter given by v≃sv_dm, with s<1, such that the lower the redshift, the smaller the s; (3) the velocity probability density functions (pdf's) are asymmetric between acceleration and deceleration events; (4) the pdf of velocity difference Δv=v(x+r)-v(x) satisfies the scaling relation for a Burgers fluid, i.e., P(Δv)=(1/r^y)F(Δv/r^y). We find the scaling function and parameters for the IGM that are applicable to the entire scale range of the samples (0.26-8 h^–1 Mpc). These properties show that the similarity mapping between the IGM and dark matter is violated on scales much larger than the Jeans length of the IGM.